Update doc/user/project/description_templates.md Issue Template Nesting

What does this MR do and why?

The description for creating an Issue Templates repository is slightly unclear. When I tried to nest the issue_templates directory under the .gitlab directory as the instructions say, it would just nest the directory under the master branch. Changing it to .gitlab/issue_templates solved this for me.

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.

typefeature