[LINTER] Escape the % char in the description of PythonFormatLinter

Summary:
The % character in the getInfoDescription() method needs to be
escaped. Otherwise it throws an exception when running arc linters.

Test Plan:

arc linters lint-python-format

Check the description string is displayed correcty.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2757