Remove unecessary `git checkout` command from README

The command is not needed, because the specified branch is already the
default branch on GitHub so will be checked out by clone automatically.

PiperOrigin-RevId: 430910549

README.md

@@ -102,13 +102,11 @@ using some ExoPlayer extension modules. It's also a suitable approach if you
 want to make local changes to ExoPlayer, or if you want to use a development
 branch.
 
-First, clone the repository into a local directory and checkout the desired
-branch:
+First, clone the repository into a local directory:
 
 ```sh
 git clone https://github.com/google/ExoPlayer.git
 cd ExoPlayer
-git checkout release-v2
 ```
 
 Next, add the following to your project's `settings.gradle` file, replacing