Fix: Adapt template to new rules (#36705)

Summary:
Recently, we changed the logic to verify the body of a PR so that it is more compatible internally and externally, in both ways. But we forgot to update the template, so right
now, all the created PR are incompatible with the internal PR checker.
The required change is to have `:` after the Changelog title.
For consistency, I added the `:` after ALL the titles.

## Changelog:

[Internal] - Changed the PR template to align it to the new rules

Pull Request resolved: https://github.com/facebook/react-native/pull/36705

Test Plan: No danger errors nor internal linter errors

Reviewed By: christophpurrer

Differential Revision: D44503519

Pulled By: cipolleschi

fbshipit-source-id: 43963c3cf774d19d6de8e86b18b684e76dd0fe87