Fix reference to action groups

[lfolco]

Description

The original text looks like it was copied from the Action Group extends article.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/verification tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[coveralls]

Coverage increased (+0.3%) to 53.574% when pulling 1039d79 on lfolco:patch-2 into 6675ee5 on magento:master.

[coveralls]

Coverage increased (+0.3%) to 53.592% when pulling 09dd487 on lfolco:patch-2 into 6675ee5 on magento:master.