docs: fix Title Case section headings in MAINTAINER-LICENSE-GUIDE.md

[amir-rezaei]

Description

This PR fixes section heading Title Case capitalization in MAINTAINER-LICENSE-GUIDE.md.

Details

Updated headers (### Common distro paths, ### Source distributions, ### Binary distributions, and ### OS package managers) to match Title Case conventions.

[charlesstaats]

Eliminating the redundant preposition and capitalizing windows -> Windows feel like worthwhile changes.

Do you have a situation where the title capitalization caused confusion? If so, we should talk to John Bowman about what capitalization convention should be preferred. (Personally, I'd go with "first word only" for ### and below, but John may have other preferences.) In the absence of a set convention, the title changes feel like churn.

[johncbowman]

I agree: first-word capitals for ### and below is better as it helps distinguish subsections from sections.