Remove reference to non existing documentation

The release management chapter in the OBS book is no where to be found.

Resolves: #458

osc/commandline.py

@@ -2993,8 +2993,7 @@ def do_release(self, subcmd, opts, *args):
         """${cmd_name}: Release sources and binaries
 
         This command is used to transfer sources and binaries without rebuilding them.
-        It requires defined release targets set to trigger="manual". Please refer the
-        release management chapter in the OBS book for details.
+        It requires defined release targets set to trigger="manual".
 
         usage:
             osc release [ SOURCEPROJECT [ SOURCEPACKAGE ] ]