compiler: correct grammar in error message

For golang/go#43200

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/278452

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-10d3dd939d4cea7f40b76f8ff82c16aa12c01188
+526037336231593939a517b7c0b2892d413adb40
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/statements.cc

@@ -4538,7 +4538,7 @@ Switch_statement::do_lower(Gogo*, Named_object*, Block* enclosing,
 					      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);
     }