rustdoc: Don't escape contents of headers

Turns out sundown has already escaped this content for us, so there's no need
for us to escape it again.

Closes #12736
auto merge of #12737 : alexcrichton/rust/issue-12736, r=brson

Turns out sundown has already escaped this content for us, so there's no need
for us to escape it again.

Closes #12736