Fix AllowedMethod parameter name

Resolves warning:

> Warning: Style/TrivialAccessors does not support AllowedMethod parameter.
>
> Supported parameters are:
>
>  - Enabled
>  - ExactNameMatch
>  - AllowPredicates
>  - AllowDSLWriters
>  - IgnoreClassMethods
>  - AllowedMethods

(Issue where bug introduced to Rubocop)[rubocop/rubocop#7469]
(Issue describing resolution)[rubocop/rubocop#7530]

config/gds-ruby-styleguide.yml

@@ -245,7 +245,7 @@ Style/TrivialAccessors:
   ExactNameMatch: true
   AllowPredicates: true
   AllowDSLWriters: false
-  AllowedMethod:
+  AllowedMethods:
   - to_ary
   - to_a
   - to_c