Update create_release_pull_request

.ci/github/create_release_pull_request

@@ -2,11 +2,11 @@
 
 set -eo pipefail
 
-# If we aren't updating an existing pull request, create one to merge develop into master.
+# If we aren't updating an existing pull request, create one to merge the release branch into master.
 if [[ -z "$PR_NUMBER" ]]; then
   pull_request_response=$(curl -s -H "Accept: application/vnd.github+json" \
     -H "Authorization: token $ACCESS_TOKEN" \
-    -X POST -d '{"title": "Release", "head": "develop", "base": "master"}' \
+    -X POST -d "{'title': 'Release', 'head': $RELEASE_BRANCH, 'base': 'master'}" \
     "https://api.github.com/repos/$REPO/pulls")
 
   message=$(echo "$pull_request_response" | jq -r 'if has("errors") then .errors[].message else null end')