Correct README pip install URL for GitHub source installs

[Copilot]

The README’s pip install command used a GitHub URL form that returns HTTP 404 in this repository context. This updates the install instruction to the working URL format so users can install directly from GitHub as documented.

• Problem

  • Quick-install docs pointed to a non-working pip Git URL (...gnn-builder.git) that fails with 404.

• Change

  • Updated the README pip command to remove the .git suffix in the GitHub source URL.

• Docs delta (exact)

  # before
  pip install git+https://github.com/sharc-lab/gnn-builder.git
  
  # after
  pip install git+https://github.com/sharc-lab/gnn-builder

• Fixes pip installation error #2