Fix some broken fragment ID encodings in rustdoc links

[eepeep]

I'm not exactly sure why they aren't decoded and considered equivalent, but the links were not working as-is for me in Chrome.

[mgeisler]

Thanks for the PR!

I feel I've fixed this before... or at least changed the percent encoding here before. I'll have to try and figure out what's going on and why some people see broken links here.

[mgeisler]

Hi @djmitche, this is similar to your #917 — did you find out why the links changed in your PR?

[djmitche]

Probably the lesson here is that we shouldn't be deep-linking to rust docs!

[mgeisler]

I rebased the commit and this should merge after the tests are done.