Fix substraction with overflow in wrong_number_of_generic_args.rs

Rarranging the substration and equality check into an addition and an equality
check is sufficient.

Algebra is cool, isn't it?

Co-authored-by: Michael Goulet <michael@errs.io>

Author: fuzzypixelz

compiler/rustc_hir_analysis/src/structured_errors/wrong_number_of_generic_args.rs

@@ -728,7 +728,8 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
         && let Some(trait_path_segment) = path.segments.get(0) {
             let num_generic_args_supplied_to_trait = trait_path_segment.args().num_generic_params();
 
-            if num_assoc_fn_excess_args == num_trait_generics_except_self - num_generic_args_supplied_to_trait {
+            if num_generic_args_supplied_to_trait + num_assoc_fn_excess_args == num_trait_generics_except_self
+            {
                 if let Some(span) = self.gen_args.span_ext()
                 && let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
                     let sugg = vec![