chore: remove mention of py_proto_library from readme

py_proto_library is meant to be imported from protobuf.

#3508 removed the relevant examples. This PR also removes the mention in main readme.

Author: shayanhoshyari

README.md

@@ -5,7 +5,7 @@
 ## Overview
 
 This repository is the home of the core Python rules -- `py_library`,
-`py_binary`, `py_test`, `py_proto_library`, and related symbols that provide the basis for Python
+`py_binary`, `py_test`, and related symbols that provide the basis for Python
 support in Bazel. It also contains package installation rules for integrating with PyPI and other indices. 
 
 Documentation for rules_python is at <https://rules-python.readthedocs.io> and in the