Update turn-howto.rst to use git clone
Update turn-howto.rst to use git clone

svn checkout is not logical for a checkout from github, so changed the checkout to "git clone". 
thanks @dbkr

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