Update README.md

Change from `git checkout` to `git switch`

README.md

@@ -63,7 +63,7 @@ Ensure that you can successfully generate a static site as above.
 
 Then, when you're ready to deploy, run the following:
 
-`git checkout deploy && git merge master`
+`git switch deploy && git merge master`
 
 This obviously assumes that you have ssh push access to the help-center-docs repo and are using `[email protected]:getsolus/help-center-docs.git` (as listed by `git config -l`).