doc: Update obsolete links to online reference

[hebasto]

Links https://bitcoin.org/en/developer-reference#... in the protocol.h are obsolete and require to be updated with a scripted-diff.

Useful skills:

bash and basic knowledge of how to contribute patches via git on GitHub.

Want to work on this issue?

For guidance on contributing, please read CONTRIBUTING.md before opening your pull request.

[ecurrencyhodler]

I'll try to take this issue. Thanks for pointing it out!

[maflcko]

Fixed in commit a787428