Fix help text, was copy&pasted from other entry

mpcdf_setup_subproject.py

@@ -12,7 +12,7 @@ import osc.cmdln
 @osc.cmdln.option('-n', '--dry-run', action="store_true",
                   help="Do not actually run anything but output the resulting XML configuration")
 @osc.cmdln.option('--only-project', action="store_true",
-                  help="Do not actually run anything but output the resulting XML configuration")
+                  help="Only change project metadata, do not iterate over packages and change their enabled repositories")
 @osc.cmdln.option('--remove-old', action="store_true", default=False,
                   help="Remove all obsolete repositories instead of only disabling builds for packages there")
 @osc.cmdln.alias("mpcdf_setup_sub")