Fix grammar in the "Contributing to Mautic" section

[adiati98]

Description

We need to fix the grammar in this section.

How to fix

1. Open the file at: /docs/contributing/contributing_to_mautic.rst
2. Remove the .. vale off and .. vale on lines in the file.
3. In the terminal, assuming you're in the docs directory, run vale contributing/contributing_to_mautic.rst command.
4. Address each vale errors, warnings, and suggestions as much as possible. You can check if they're fixed by running the vale teams/education_team.rst command again.

Note

Please read and follow our Style Guide when fixing/updating writing style and grammar in our docs.