Docs: Fix link to "Compile-time Format String Checks" section

[Pokechu22]

The link in the safety section goes to a nonexistant c.fmt anchor on the API page, when it should go to the section for the Compile-Time Format String Checks. I don't think the old anchor ever worked (at least, no such anchor exists in the oldest archived copy of the page).

I haven't actually tested that this link works now because I haven't set up building the docs locally, but I'm almost certain that this is the correct way of formatting a link.

[vitaut]

Thank you!