fix comment to make update stack param Timeout optional
fix comment to make update stack param Timeout optional

pratikmallya/gophercloud,feiskyer/gophercloud,DSpeichert/gophercloud,jarosser06/gophercloud,larsbutler/gophercloud,timbyr/gophercloud,timbyr/gophercloud,clintonskitson/gophercloud,clintonskitson/gophercloud,jamiehannaford/gophercloud,larsbutler/gophercloud,trumant/gophercloud,trumant/gophercloud,dudymas/gophercloud,carolynvs/gophercloud,jarosser06/gophercloud,pratikmallya/gophercloud,jamiehannaford/gophercloud,dudymas/gophercloud,carolynvs/gophercloud,feiskyer/gophercloud,DSpeichert/gophercloud,rackspace/gophercloud,sti-jans/gophercloud,sti-jans/gophercloud,rackspace/gophercloud