docs: fix indentation of nested lists

[YDX-2147483647]

Nested lists require 4 spaces of indent in Python Markdown, which differs from GitHub Flavored Markdown.

This PR fixes the indent in https://guts.github.io/mkdocs-rss-plugin/#credits.

Relates-to: Python-Markdown/markdown#3

Besides, rg ' -' ./docs/ --type=md --context 5 finds another nested list, but its indentation is correct.

mkdocs-rss-plugin/docs/configuration.md

Lines 416 to 418 in 8935f6c

	The timezones data depends on the Python version used to build:
	- for Python >= 3.9, it uses the standard library and ships [tzdata](https://pypi.org/project/tzdata/) only on Windows which do not provide such data
	- for Python < 3.9, [pytz](https://pypi.org/project/pytz/) is shipped.