go/types: fix error message for consistency

Follow-up on https://golang.org/cl/271706 .
(Missed a review comment.)

Change-Id: Ibff542f43d721600a2452907c0a20941961e793f
Reviewed-on: https://go-review.googlesource.com/c/go/+/271766
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index 5bf9c81..4e19f30 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -890,7 +890,7 @@
 			// TODO(gri) We should report exactly what went wrong. At the
 			//           moment we don't have the (go/constant) API for that.
 			//           See also TODO in go/constant/value.go.
-			check.errorf(atPos(e.OpPos), _InvalidConstVal, "constant result not representable")
+			check.errorf(atPos(e.OpPos), _InvalidConstVal, "constant result is not representable")
 			// TODO(gri) Should we mark operands with unknown values as invalid?
 		}
 		// Typed constants must be representable in