Remove duplicated command

lukabot · lukabot · commit 7745022262ad · 2017-10-24T11:14:00.000+09:00
diff --git a/README.md b/README.md
@@ -563,12 +563,6 @@ git clean -f
 git clean -f -d
 ```
 
-
-__Alternatives:__
-```sh
-git clean -df
-```
-
 ## Update all the submodules
 ```sh
 git submodule foreach git pull
