Use git submodule update --init --recursive

In a case of nested submodules, like swagger-ui inside mgmt inside osv,
only the --recursive update all the submodules, all the way down.

Signed-off-by: Tzach Livyatan <tzach@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

README.md

@@ -48,7 +48,7 @@ the first version to fully comply with the C++11 standard.
 Make sure all git submodules are up-to-date:
 
 ```
-git submodule update --init
+git submodule update --init --recursive
 ```
 
 Finally, build everything at once: