Pull request message about commit message format is incorrect

[gshimansky]

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04):

Ubuntu 20.04.1

• Modin version (modin.__version__):

0.8.1.1+4.gbd8f73a

When PR is created, developer is asked a question whether message format confirms to format described in https://modin.readthedocs.io/en/latest/contributing.html. The problem is that this page doesn't exist. There is a page https://modin.readthedocs.io/en/latest/developer/contributing.html but it doesn't mention anything about special message format.

[anmyachev]

#2193 solves this problem.