[BUGFIX] modify VCS repo for t3g/hubspot-forms

Since bitbucket.typo3.com does not exist anymore,
and the package "t3g/hubspot-forms" has been moved
to GitHub, the URL for the VCS section in composer.json
has been adapted.

Process:
- Manually updated the URL (no composer config repositories
  as the current composer.json was modified manually already)
- ran composer update t3g/hubspot-forms

Resolves: 473