fix grammar and wording

Author: Jendrik Wenke

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -2087,11 +2087,11 @@ object messages {
         // compare the signatures when both symbols represent methods
         decl.signature.matchDegree(previousDecl.signature) match {
           /* case Signature.NoMatch => // can't happen because decl.matches(previousDecl) is checked before reporting this error */
-          case Signature.ParamMatch => "\nOverloads with equal parameter types but different return types are not allowed."
-          case _ /* Signature.FullMatch */ => "\nThe definitions have the same signature after erasure."
+          case Signature.ParamMatch => "\nOverloads with matching parameter types are not allowed."
+          case _ /* Signature.FullMatch */ => "\nThe definitions have matching type signatures after erasure."
         }
       } else ""
-      hl"${decl.showLocated} is already defined as ${previousDecl.showDcl} in line ${previousDecl.pos.line + 1}." + details
+      hl"${decl.showLocated} is already defined as ${previousDecl.showDcl} at line ${previousDecl.pos.line + 1}." + details
     }
     val explanation = ""
   }