Fixing signurl help synopsis and examples
Fixing signurl help synopsis and examples

craigcitro/gsutil,BrandonY/gsutil,GoogleCloudPlatform/gsutil,GoogleCloudPlatform/gsutil,ttiurani/gsutil,mattdr/gsutil,dimfeld/gsutil,chriskuehl/gsutil-debian,dtjackson/gsutil,fishjord/gsutil