Fix broken link on Clang documentation page

While browsing the latest [clang manual](https://clang.llvm.org/docs/ReleaseNotes.html), I came across this warning and clicked, but it leads to a broken page, apparently because it's missing the trailing `/`. Seems to be the only place where it's missing in the whole of LLVM monorepo.

{F26690053}
{F26690057}

Reviewed By: brenoguim, royjacobson

Differential Revision: https://reviews.llvm.org/D145047