Fix `docker rmi` tab completion

This is exactly what happens on tab completion for docker rmi. This commit
fixes that.

```sh
$ docker rmi <hit TAB>
_arguments:comparguments:312: invalid argument: __docker_images
_arguments:comparguments:312: invalid argument: __docker_images
_arguments:comparguments:312: invalid argument: __docker_images
```

plugins/docker/_docker

@@ -166,7 +166,7 @@ __rm() {
 
 __rmi() {
     _arguments \
-        '(-f,--force=)'{-f,--force=}'[Force]' \
+        '(-f,--force=)'{-f,--force=}'[Force]'
     __docker_images
 }