Add '--recursive' to git submodule update --init (#10180)

See edit here:  https://github.com/pytorch/executorch/pull/10082/files

Co-authored-by: lucylq <[email protected]>

Author: pytorchbot

docs/source/using-executorch-building-from-source.md

@@ -113,7 +113,7 @@ Or alternatively, [install conda on your machine](https://conda.io/projects/cond
 > # From the root of the executorch repo:
 > ./install_executorch.sh --clean
 > git submodule sync
-> git submodule update --init
+> git submodule update --init --recursive
 > ```
 
 ## Build ExecuTorch C++ runtime from source