_navbar.html: Use relative path to index
_navbar.html: Use relative path to index

Issue #71.

ChrisCummins/chriscummins.github.io,ChrisCummins/chriscummins.github.io,ChrisCummins/chriscummins.cc,ChrisCummins/chriscummins.cc,ChrisCummins/chriscummins.github.io,ChrisCummins/chriscummins.github.io,ChrisCummins/chriscummins.github.io,ChrisCummins/chriscummins.cc