Update getting_started.md first line wording

What does this MR do and why?

This merge request updates the first line of text on the MR page in the Docs to (hopefully) improve readability. (And it was a part of my onboarding requirements to create a small MR to a Docs page)

Screenshots or screen recordings

Previously:

Updated to read:
A merge request (MR) is the basis of GitLab as a tool for code collaboration and version control.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.