extensions/translation: fix the link to the main translation page

Fixes an incorrect link on the extension translation page:

• Go to: https://docs.civicrm.org/dev/en/latest/extensions/translation/
• click the link on the first sentence, i.e. "all the best practices described in the Internationalisation for Developers page apply"
• it goes to the docs home page, instead of the dev-translation page.