Update solution for stashing exercise

Using `git stash push` instead of `git stash` as it is clearer, although both work.

Author: dianaiusan

exercises-solutions/4.commits/3.stash/README.md

@@ -43,7 +43,7 @@ no changes added to commit (use "git add" and/or "git commit -a")
 Save the changes in the working directory to the stash:
 
 ```shell
-$ git stash
+$ git stash push
 Saved working directory and index state WIP on master: 94e9a51 Add directions
 ```