tcfail175: improved error message slightly

testsuite/tests/ghc-regress/typecheck/should_fail/tcfail175.stderr

@@ -2,3 +2,5 @@
 tcfail175.hs:11:0:
     Couldn't match expected type `a'
 	   against inferred type `String -> String -> String'
+    The equation(s) for `evalRHS' have three arguments,
+    which does not match its type