Update documentation for submdule sync

Summary:
Subodule sync and update is necessary whenver commit hash of any submodule
changes. Thus it is safe to do git submodule update anytime you pull from
upstream

Reviewed By: mcr229

Differential Revision: D48357048

fbshipit-source-id: 29622668e56a9aefa0004b33b577898f66ae8841

Author: kimishpatel

docs/website/docs/tutorials/00_setting_up_executorch.md

@@ -93,11 +93,12 @@ git clone https://github.com/pytorch/executorch.git
 git clone [email protected]:pytorch/executorch.git
 ```
 
-Ensure that git has fetched the submodules. This is only necessary after
-cloning.
+Ensure that git has fetched and updated the submodules. This is necessary anytime
+commit hash of any of the submodules changes. Thus it is safe and necessary at times to apply this step after you pull changes from upstream.
 
 ```bash
 cd executorch
+git submodule sync
 git submodule update --init
 ```