update missing flag message

README.md

@@ -18,7 +18,7 @@ In order to use the slide deck, you must have:
     ```bash
     $ git clone --recurse-submodules ssh://git@git-r3lab-server.uni.lu:8022/<yourUsername>/presentations.git
     ```
-- Be sure all submodules are installed and updated:
+- If the `--recurse-submodules` flag has been omitted, run this command to check for the missing modules and update them:
     ```bash
     $ git submodule update --init --recursive
     ```