Fix error message when trait method ends with wrong token

When parsing a trait function, the function must end with either `;` or
`{` (signifying a default implementation). The error message incorrectly
stated that it must be `;` or `}`.

Fixes #6610.

src/libsyntax/parse/parser.rs

@@ -953,7 +953,7 @@ impl Parser {
               _ => {
                     p.fatal(
                         fmt!(
-                            "expected `;` or `}` but found `%s`",
+                            "expected `;` or `{` but found `%s`",
                             self.this_token_to_str()
                         )
                     );