Fix broken Doxygen link in README

[ckeshava]

After porting the codebase to XRPLF Github repository, the link to Doxygen documentation is broken. Previously, it pointed to https://rippled.github.io/rippled/. It needs to be reconfigured for https://xrplf.github.io/rippled/

I'm referring to the README file at the root of the repository - README.md