Suppress RuboCop's offense

Follow rubocop/rubocop#8204.

This commit suppresses the following RuboCop's offense.

```console
% bundle exec rubocop
(snip)

Offenses:

lib/rubocop/cop/rails/save_bang.rb:141:57: W:
Lint/RedundantCopDisableDirective: Unnecessary disabling of
Metrics/PerceivedComplexity.
        # rubocop:disable Metrics/CyclomaticComplexity,
        Metrics/PerceivedComplexity
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^

181 files inspected, 1 offense detected
RuboCop failed!
```

lib/rubocop/cop/rails/save_bang.rb

@@ -138,7 +138,7 @@ def check_assignment(assignment)
           add_offense_for_node(node, CREATE_MSG)
         end
 
-        # rubocop:disable Metrics/CyclomaticComplexity, Metrics/PerceivedComplexity
+        # rubocop:disable Metrics/CyclomaticComplexity
         def on_send(node)
           return unless persist_method?(node)
           return if return_value_assigned?(node)
@@ -150,7 +150,7 @@ def on_send(node)
 
           add_offense_for_node(node)
         end
-        # rubocop:enable Metrics/CyclomaticComplexity, Metrics/PerceivedComplexity
+        # rubocop:enable Metrics/CyclomaticComplexity
         alias on_csend on_send
 
         def autocorrect(node)