Fix urls in documentation

[FZambia]

Hello! It was great to find your project:) Looked through documentation and found that some urls forward to 404 pages, here is fix. I can't say exactly that I fixed them right though.

[coveralls]

Coverage remained the same at 90.367% when pulling 5b630e9 on FZambia:patch-1 into 71332c3 on raphaelgyory:master.

[raphaelgyory]

Hi Alexandr! Thank you for your feedback. Indeed, the links were broken.
Ps: thanks for your work on Centrifugo, it's great and very helpful.

[FZambia]

Thanks, glad it helps!:)