Adds an example of git-repository-url in book.toml

[ironicbadger]

#802 seemingly didn't remember to update the docs with this option, this PR address it.

[ehuss]

I'm not sure I understand. That example is just an small sample of some settings, it doesn't include all of them. Why add this particular setting? This setting is documented in https://rust-lang.github.io/mdBook/format/config.html#html-renderer-options, and at the end of that section there is a complete TOML example that includes git-repository-url.

[ironicbadger]

Apparently reading is hard. I thought I'd checked thoroughly but clearly not.

Feel free to close this one out.

[ehuss]

Sure, no problem!