compiler: correct grammar in error message

For golang/go#43200

Change-Id: I7f102bfdd5a2578e59292be4c84e11df107f65c6
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/278452
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed