Fixed typo in reverse_proxy.md

[PracticalRyan]

Fixed typo to Authelia's forwarded-headers page

[francislavoie]

I think that was probably legitimately the URL at the time, I doubt I typed that out by hand :sweat_smile: but Authelia probably fixed the typo since. Thanks!

[mholt]

Oh haha, that's a good point. Interesting!