Suppress RuboCop's offense

Follow rubocop/rubocop#8122.

This commit suppresses the following RuboCop's offense.

```console
% bundle exec rake
(snip)

Offenses:

lib/rubocop/cop/mixin/index_method.rb:122:80: W:
Lint/RedundantCopDisableDirective: Unnecessary disabling of
Metrics/BlockLength.
      Autocorrection = Struct.new(:match, :block_node, :leading,
      :trailing) do # rubocop:disable Metrics/BlockLength
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

187 files inspected, 1 offense detected, 1 offense auto-correctable
RuboCop failed!
```

lib/rubocop/cop/mixin/index_method.rb

@@ -119,7 +119,7 @@ def noop_transformation?
       end
 
       # Internal helper class to hold autocorrect data
-      Autocorrection = Struct.new(:match, :block_node, :leading, :trailing) do # rubocop:disable Metrics/BlockLength
+      Autocorrection = Struct.new(:match, :block_node, :leading, :trailing) do
         def self.from_each_with_object(node, match)
           new(match, node, 0, 0)
         end