Change "html" to "HTML" in atomlib.php

[man4toman]

Follow #50473, there is a string in wp-includes\atomlib.php

https://core.trac.wordpress.org/browser/tags/5.4/src/wp-includes/atomlib.php#L217

Invalid content in element found. Content must not be of type text or html if it contains markup.

[burhandodhy]

Replace html with HTML

[SergeyBiryukov]

Thanks for the ticket!

Similarly to class-pclzip.php in #49163, atomlib.php used to be treated as an external library and was previously excluded from any documentation or coding standard fixes. We only modify third-party libraries when it's needed to fix a bug, like in [37461], [38883], or [39591].

That said, looking at the commit history, there were some string changes in the past.

I think it's safe to treat atomlib.php as "adopted" library rather than external. While I don't feel strongly about making this change, I don't see any harm in it either.

[SergeyBiryukov]

Related: #51306

Trac ticket: https://core.trac.wordpress.org/ticket/50498

[rehanali]

Improves the documentation.Replaced the "html" with "HTML".