Update README.md

Correction to git checkout command for release branch.

README.md

@@ -40,7 +40,7 @@ With your distro's package manager, make sure you have installed `gcc`, `make`,
 *If the build fails for you, please report the issue with a detailed error message to help the portability of Rack.*
 
 Clone this repository and `cd` into it.
-If you would like to build a previous version of Rack instead of the master branch, check out the desired tag with `git checkout v0.4.0` for example.
+If you would like to build a previous version of Rack instead of the master branch, check out the desired tag with `git checkout -b v0.4.0` for example.
 
 Clone submodules.