add header line

Author: gmiller-mdb

source/api.txt

@@ -34,6 +34,7 @@ This ``curl`` example retrieves database users for a project:
    curl --user "{PUBLIC-KEY}:{PRIVATE-KEY}" --digest \
      --header "Accept: application/json" \
      --header "Content-Type: application/json" \
+     --header "Accept: application/vnd.atlas.2023-01-01+json" \ # update date to desired API version
      --include \
      --request GET "https://cloud.mongodbgov.com/api/atlas/v2/groups/{PROJECT-ID}/databaseUsers?pretty=true"