mergify: add spellcheck to required checks

Problem: The spelling check is missing from required checks
in the mergify config.

Add the missing required check.

Author: chu11

.mergify.yml

@@ -3,6 +3,7 @@ queue_rules:
     conditions:
       - base=master
       - status-success="validate commits"
+      - status-success="spelling"
       - status-success="flux-sched check"
       - status-success="flux-accounting check"
       - status-success="python linting"