Fix gitter link

This fixes a bug introduced by #34: changing the micrositeGithubRepo
to point to freechipsproject/www.chisel-lang.org (to point at the
correct repository for editing) broke the Gitter channel link which
should point at freechipsproject/chisel3. This corrects that.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>