Change apostrophe in error message

I'm not sure if this was intentionally left with `'` instead of ` in the closing apostrophe, but just in case it was not I made the PR.
Scala CLA has been signed.

Author: egulias

src/main/scala/scala/util/parsing/combinator/Parsers.scala

@@ -565,7 +565,7 @@ trait Parsers {
    *  @return a `tParser` that succeeds if `e` is the next available input.
    */
 
-  implicit def accept(e: Elem): Parser[Elem] = acceptIf(_ == e)("`"+e+"' expected but " + _ + " found")
+  implicit def accept(e: Elem): Parser[Elem] = acceptIf(_ == e)("`"+e+"` expected but " + _ + " found")
 
   /** A parser that matches only the given list of element `es`.
    *

src/test/scala/scala/util/parsing/combinator/PackratParsersTest.scala

@@ -122,7 +122,7 @@ class PackratParsersTest {
       val failure = parseResult.asInstanceOf[Failure]
       assertEquals(expectedFailureMsg, failure.msg)
     }
-    assertFailure("``b'' expected but `c' found", "a a a a b b b c c c c")
+    assertFailure("``b'` expected but `c' found", "a a a a b b b c c c c")
     assertFailure("end of input", "a a a a b b b b c c c")
   }