chore: update comment to clarify that JSON locale files are supported

[sho-he]

Hi.
This pull request updates the documentation for the I18n library to clarify that translation files can also be in JSON format.
Since, in Base, the parent class of SimpleBackend, the following is defined:

i18n/lib/i18n/backend/base.rb

Line 17 in f2fb6a5

loaded_translations = load_file(filename)

i18n/lib/i18n/backend/base.rb

Line 276 in f2fb6a5

def load_json(filename)

handles JSON files as locale files, I would like to explicitly mention JSON support in the configuration and documentation as well.

I apologize for bringing up such a minor detail, but I appreciate your attention to it.

• Documentation update:
  • The comment in lib/i18n/config.rb now specifies that translation files can be named .json and contain JSON data, expanding the list of supported formats for translation files.