Fix spelling.

README.md

@@ -92,7 +92,7 @@ adduser $new_user_name
 
 The user can be added to a group with.
 ```sh
-usermod -aG $grou_name $new_user_name
+usermod -aG $group_name $new_user_name
 ```
 
 **Changing the ownership of a file or directory**