Update turn-howto.rst to use git clone (2)
Update turn-howto.rst to use git clone (2)

Not logical to use svn checkout against a github repo, so changed to git clone. 

Signed-off-by: Rick Cogley <72aa636c2db4676094de050d0f2b0246fb87e677@esolia.co.jp>
TribeMedia/synapse,TribeMedia/synapse,matrix-org/synapse,TribeMedia/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,TribeMedia/synapse,TribeMedia/synapse,matrix-org/synapse,matrix-org/synapse