readme: added submodule update precision

README.md

@@ -168,9 +168,9 @@ git pull figli master
 # then update all submodules
 git submodule update --recursive --checkout
 # or only one
-git submodule update app/
+git submodule update --checkout app/
 # or
-git submodule update api/
+git submodule update --checkout api/
 make api_maj_config
 ```