Headers should link to the #c-heading

[dtolnay]

For example clicking on the "Macro syntax is evocative of the output" currently links to https://rust-lang-nursery.github.io/api-guidelines/macros.html#input-syntax-is-evocative-of-the-output-c-evocative. Instead it should link to https://rust-lang-nursery.github.io/api-guidelines/macros.html#c-evocative as the checklist does. This would allow us to improve the wording of guideline titles without breaking links.