docs: fix deprecated git:// URL in installation guide

[TejasAmle]

What

The "Install from GitHub" section of the installation docs uses the git:// protocol:

pip install -U git+git://github.com/python-cmd2/cmd2.git

GitHub disabled the unauthenticated Git protocol (git://) on March 15, 2022. Anyone following this instruction today will see an error like:

fatal: remote error: The unauthenticated git protocol on port 9418 is no longer supported.

Fix

Replace git:// with https://:

pip install -U git+https://github.com/python-cmd2/cmd2.git

Verification

The fixed command works correctly and pip is able to install cmd2 directly from the GitHub main branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.59%. Comparing base (0786a4e) to head (2a35236).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1675      +/-   ##
==========================================
+ Coverage   99.57%   99.59%   +0.01%     
==========================================
  Files          23       23              
  Lines        5677     5677              
==========================================
+ Hits         5653     5654       +1     
+ Misses         24       23       -1     

Flag	Coverage Δ
unittests	99.59% <ø> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

[tleonhardt]

@TejasAmle Hey Tejas, thanks for catching this and taking the time to fix it! Much appreciated.