docs: fix a typo in registry_mirror.md

Signed-off-by: Zhang Wei <zhangwei555@huawei.com>

docs/sources/articles/registry_mirror.md

@@ -38,7 +38,7 @@ For example, if your mirror is serving on `http://10.0.0.2:5000`, you would run:
 **NOTE:**
 Depending on your local host setup, you may be able to add the
 `--registry-mirror` options to the `DOCKER_OPTS` variable in
-`/etc/defaults/docker`.
+`/etc/default/docker`.
 
 ### Step 2: Run the local registry mirror