Add a `cd` command to ExoPlayer clone instructions

With this missing, the `checkout` command errors with:
$ git checkout release-v2
fatal: not a git repository (or any of the parent directories): .git
PiperOrigin-RevId: 270670796

README.md

@@ -107,6 +107,7 @@ branch:
 
 ```sh
 git clone https://github.com/google/ExoPlayer.git
+cd ExoPlayer
 git checkout release-v2
 ```