Clarify git user.name usage

What does this MR do and why?

Contributes to #432764 (closed)

Problem

The current version of the documentation is confusing. It might be read as user needs to provide their GitLab username instead of their name in git configuration.

See git documentation: https://git-scm.com/book/en/v2/Customizing-Git-Git-Configuration

Solution

Replace username with full name in the documentation

MR acceptance checklist

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