Fix documentation (projects guide) regarding adding a git dependency

[VolkerH]

Summary

This is a trivial, one line documentation change that fixes the following documentation bug.

The current documentation suggests this for adding a git dependency

# Add a git dependency

uv add requests --git https://github.com/psf/requests

Executing what is suggested with uv version 0.4.18 results in this error message

uv add requests --git https://github.com/psf/requests
error: unexpected argument '--git' found

The working approach is to add the git depency like this:

uv add git+https://github.com/psf/requests

Test Plan

I manually tested the command suggested currently in the guide against my change.

[charliermarsh]

Thx!