docs/getting-started: fix repo-cloning instructions

 - Fix a discrepancy between repo and cloned directory names.
 - Let rust-bootstrap handle rust submodules instead of cloning them by
   default, such that only needed submodules are cloned (notably, the
   gcc codegen backend is not used by rex).

Fixes: 7b8ea9e ("docs: add getting-started instructions")
Signed-off-by: Jinghao Jia <jinghao7@illinois.edu>

Author: jinghao-jia

docs/getting-started.md

@@ -34,9 +34,9 @@ standard kernel build dependencies.
 Clone this repo and its submodules:
 
 ```bash
-git clone https://github.com/rex-rs/rex.git
+git clone https://github.com/rex-rs/rex.git rex-kernel
 cd rex-kernel
-git submodule update --init --recursive --progress
+git submodule update --init --progress
 ```
 
 If you are using Nix, the following additional step is required.