Adjust mentions & usage of n_ & n__ in i18n externalization docs

- mentions n__ along with n_ in regards to proper usage
- fixes a related Ruby example which had n__ instead of n_

doc/development/i18n/externalization.md

@@ -363,7 +363,7 @@ use `%{created_at}` in Ruby but `%{createdAt}` in JavaScript. Make sure to
   // => When x == 2: 'Last 2 days'
   ```
 
-The `n_` method should only be used to fetch pluralized translations of the same
+The `n_` and `n__` methods should only be used to fetch pluralized translations of the same
 string, not to control the logic of showing different strings for different
 quantities. Some languages have different quantities of target plural forms.
 For example, Chinese (simplified) has only one target plural form in our
@@ -376,7 +376,7 @@ For example, use this:
 if selected_projects.one?
   selected_projects.first.name
 else
-  n__("Project selected", "%d projects selected", selected_projects.count)
+  n_("Project selected", "%d projects selected", selected_projects.count)
 end
 ```