Fix a typo for Rails/BelongsTo

This PR fixes a typo for `Rails/BelongsTo`.

```console
% misspell -i enviromnent .
lib/rubocop/cop/rails/belongs_to.rb:22:53: "overriden" is a misspelling of "overridden"
manual/cops_rails.md:187:45: "overriden" is a misspelling of "overridden"
```

Author: koic

lib/rubocop/cop/rails/belongs_to.rb

@@ -19,7 +19,7 @@ module Rails
       # In the case that the developer is doing `required: false`, we
       # definitely want to autocorrect to `optional: true`.
       #
-      # However, without knowing whether they've set overriden the default
+      # However, without knowing whether they've set overridden the default
       # value of `config.active_record.belongs_to_required_by_default`, we
       # can't say whether it's safe to remove `required: true` or whether we
       # should replace it with `optional: false` (or, similarly, remove a

manual/cops_rails.md

@@ -184,7 +184,7 @@ From the release notes:
 In the case that the developer is doing `required: false`, we
 definitely want to autocorrect to `optional: true`.
 
-However, without knowing whether they've set overriden the default
+However, without knowing whether they've set overridden the default
 value of `config.active_record.belongs_to_required_by_default`, we
 can't say whether it's safe to remove `required: true` or whether we
 should replace it with `optional: false` (or, similarly, remove a