Fixes homepage broken link to existing_instrumentations
Fixes homepage broken link to existing_instrumentations

https://twitter.com/galderz/status/707566090394673156
openzipkin/openzipkin.github.io,basvanbeek/openzipkin.github.io,basvanbeek/openzipkin.github.io,basvanbeek/openzipkin.github.io,openzipkin/openzipkin.github.io,openzipkin/openzipkin.github.io