doc/start/get-involved: fix links

ERROR: /srv/autobuild-ceph/gitbuilder.git/build/doc/start/get-involved.rst:33: Unknown target name: "tracker".
ERROR: /srv/autobuild-ceph/gitbuilder.git/build/doc/start/get-involved.rst:43: Unknown target name: "support".

Signed-off-by: Sage Weil <sage@inktank.com>