Fix PlainPrinter.scala: incorect type pararameters

What it prints might be counter intuitive at first, but is correct
The clearest example is in i5345, where otherwise the printed output
doesn't correspond to the declaration

Author: Sporarum

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -223,7 +223,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
       case tp: PolyType =>
         changePrec(GlobalPrec) {
           "[" ~ paramsText(tp) ~ "]" ~ lambdaHash(tp) ~
-          (Str(" => ") provided !tp.resultType.isInstanceOf[MethodType]) ~
+          (Str(": ") provided !tp.resultType.isInstanceOf[MethodType]) ~
           toTextGlobal(tp.resultType)
         }
       case AnnotatedType(tpe, annot) =>

compiler/test-resources/repl/i5345

@@ -1,2 +1,2 @@
 scala>  def x[A, B]: Conversion[A, B] = _ => ???
-def x[A, B] => Conversion[A, B]
+def x[A, B]: Conversion[A, B]

language-server/test/dotty/tools/languageserver/CompletionTest.scala

@@ -610,7 +610,7 @@ class CompletionTest {
     code"""object Foo
           |extension (foo: Foo.type) def xxxx[A]: Int = 1
           |object Main { Foo.xx${m1} }""".withSource
-      .completion(m1, Set(("xxxx", Method, "[A] => Int")))
+      .completion(m1, Set(("xxxx", Method, "[A]: Int")))
   }
 
   @Test def completeExtensionMethodWithParameterAndTypeParameter: Unit = {
@@ -729,7 +729,7 @@ class CompletionTest {
           |given Baz with {}
           |extension [A](using bar: Bar)(a: A)(using baz: Baz) def xxxx[B]: Either[A, B] = Left(a)
           |object Main { 123.xx${m1} }""".withSource
-      .completion(m1, Set(("xxxx", Method, "(using baz: Baz): [B] => Either[Int, B]")))
+      .completion(m1, Set(("xxxx", Method, "(using baz: Baz)[B]: Either[Int, B]")))
   }
 
   @Test def completeExtensionMethodWithTypeBounds: Unit = {
@@ -739,7 +739,7 @@ class CompletionTest {
           |extension [A >: Bar](a: A) def xxxx[B <: a.type]: Either[A, B] = Left(a)
           |val foo = new Foo {}
           |object Main { foo.xx${m1} }""".withSource
-          .completion(m1, Set(("xxxx", Method, "[B <: (foo : Foo)] => Either[Foo, B]")))
+          .completion(m1, Set(("xxxx", Method, "[B <: (foo : Foo)]: Either[Foo, B]")))
   }
 
   @Test def completeInheritedExtensionMethod: Unit = {

tests/neg-custom-args/fatal-warnings/i9408a.check

@@ -9,7 +9,7 @@
 -- Error: tests/neg-custom-args/fatal-warnings/i9408a.scala:26:20 ------------------------------------------------------
 26 |  val length: Int = "qwerty" // error
    |                    ^^^^^^^^
-   |The conversion (Test5.implicitLength : [A] => String => Int) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.
+   |The conversion (Test5.implicitLength : [A]: String => Int) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.
 -- Error: tests/neg-custom-args/fatal-warnings/i9408a.scala:31:20 ------------------------------------------------------
 31 |  val length: Int = "qwerty" // error
    |                    ^^^^^^^^
@@ -21,4 +21,4 @@
 -- Error: tests/neg-custom-args/fatal-warnings/i9408a.scala:59:2 -------------------------------------------------------
 59 |  123.foo // error
    |  ^^^
-   |The conversion (Test11.a2foo : [A] => A => Test11.Foo) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.
+   |The conversion (Test11.a2foo : [A]: A => Test11.Foo) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.