Fix keys for unpermitted_parameters logging [ci skip]

https://github.com/rails/rails/blob/7893428113c2536d1d58cad32999c9099787fa6f/actionpack/lib/action_controller/metal/strong_parameters.rb#L973-L974

Originally discovered by @hidr0:
https://github.com/rails/rails/pull/41809/files#r653271964

Author: zzak

guides/source/active_support_instrumentation.md

@@ -282,7 +282,7 @@ INFO. Additional keys may be added by the caller.
 
 | Key           | Value                                                                 |
 | ------------- | --------------------------------------------------------------------- |
-| `:key`        | The unpermitted keys                                                  |
+| `:keys`       | The unpermitted keys                                                  |
 | `:context`    | Hash with the following keys: :controller, :action, :params, :request |
 
 Action Dispatch