Minor English fix
CONTRIBUTING.md would use the phrase "those guidelines" to introduce a
list. In general, the word "these" is used to refer to something that is
being introduced or has recently been introduced, and "those" is used to
refer to something that had been previously introduced.
This change also makes CONTRIBUTING.md consistent with the
documentation. The documentation guidelines at
https://docs.godotengine.org/en/latest/community/contributing/documentation_guidelines.html
use "these" to introduce a list and its singular form, "this", to
introduce items.
Jason Yundt