Fix GitHub relative SVGs not rendering properly

[raftario]

Fixes #1851 where SVGs from GitHub aren't rendered in READMEs if they use relative links.

First PR, so if I did something wrong point it out!

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @sgrif (or someone else) soon.

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[fundon]

Need merge it.

[carols10cents]

Looks great! I'm going to merge later this afternoon after a change I need to deploy by itself. Thank you so much!

[carols10cents]

@bors r+

[bors]

📌 Commit b4d1464 has been approved by carols10cents

[bors]

⌛ Testing commit b4d1464 with merge 150ba49...

[bors]

☀️ Test successful - checks-travis
Approved by: carols10cents
Pushing 150ba49 to master...