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>
diff --git a/go/statements.cc b/go/statements.cc
index af82f36..398b8fd 100644
--- a/go/statements.cc
+++ b/go/statements.cc
@@ -4538,7 +4538,7 @@
 					      Type::make_nil_type(), NULL))
     {
       go_error_at(this->val_->location(),
-		  "cannot switch on value whose type that may not be compared");
+		  "cannot switch on value whose type may not be compared");
       return Statement::make_error_statement(loc);
     }