Update SyntaxHighlighting.scala

Author: kcsongor

src/dotty/tools/dotc/printing/SyntaxHighlighting.scala

@@ -4,7 +4,6 @@ package printing
 
 import parsing.Tokens._
 import scala.annotation.switch
-import scala.collection.mutable
 import scala.collection.mutable.StringBuilder
 
 /** This object provides functions for syntax highlighting in the REPL */
@@ -39,7 +38,7 @@ object SyntaxHighlighting {
   def apply(chars: Iterable[Char]): Vector[Char] = {
     var prev: Char = 0
     var remaining  = chars.toStream
-    val newBuf     = new mutable.StringBuilder
+    val newBuf     = new StringBuilder
 
     @inline def keywordStart =
       prev == 0 || prev == ' ' || prev == '{' || prev == '(' || prev == '\n'
@@ -268,5 +267,4 @@ object SyntaxHighlighting {
 
     newBuf.toVector
   }
-
 }