Update getting-started-setup.md and fix missing command (#7438)

Update getting-started-setup.md

Fixed: Error: /home/i/executorch/cmake-out/cmake-out is not a directory

Author: arkylin

docs/source/getting-started-setup.md

@@ -211,6 +211,9 @@ The ExecuTorch repo uses CMake to build its C++ code. Here, we'll configure it t
   ./install_executorch.sh --clean
   (mkdir cmake-out && cd cmake-out && cmake ..)
 
+  # Go to work directory.
+  cd ..
+
   # Build the executor_runner target
   cmake --build cmake-out --target executor_runner -j9
   ```