Fix option name in error message.

cmd_mirror.go

@@ -185,7 +185,7 @@ func aptlyMirrorDrop(cmd *commander.Command, args []string) error {
 				fmt.Printf(" * %s\n", snapshot)
 			}
 
-			return fmt.Errorf("won't delete mirror with snapshots, use --force to override")
+			return fmt.Errorf("won't delete mirror with snapshots, use -force to override")
 		}
 	}
 

system/t04_mirror/DropMirror2Test_gold

@@ -1,3 +1,3 @@
 Mirror `wheezy-main` was used to create following snapshots:
  * [wheez]: Snapshot from mirror [wheezy-main]: http://mirror.yandex.ru/debian/ wheezy
-ERROR: won't delete mirror with snapshots, use --force to override
+ERROR: won't delete mirror with snapshots, use -force to override