README.md: use HTTPS for pip install (#29)

Support for the unencrypted Git protocol was dropped by GitHub:
https://github.blog/2021-09-01-improving-git-protocol-security-github/

Author: tttapa

README.md

@@ -9,7 +9,7 @@ be referenced to generate docstrings in pybind11 binding code.
 To install the latest development version:
 
 ```bash
-python -m pip install git+git://github.com/pybind/pybind11_mkdoc.git@master
+python -m pip install git+https://github.com/pybind/pybind11_mkdoc.git@master
 ```
 
 ## Usage