fix branch ref in script

.prepare_docs

@@ -26,7 +26,7 @@ redoc-cli bundle ${API_SPEC_FILE} -o ${redoc_bundle_file}
 
 # Check out current stuff from gh-pages (we'll append to it)
 git fetch origin 'refs/heads/gh-pages:refs/heads/gh-pages'
-git checkout origin/gh-pages -- docs
+git checkout gh-pages -- docs
 
 # Create the docs bundle for our ref.  This will be the redoc bundle + a
 # snapshot of the OpenAPI spec