Update onthisday spec
Update onthisday spec

Follow-up from https://github.com/wikimedia/restbase/pull/1000#issuecomment-394802524.

Reconciles this check with mobileapps to aid in diagnosing alerts.

See also https://gerrit.wikimedia.org/r/#/c/437636/ on the mobileapps side.

wikimedia/restbase,cscott/restbase,wikimedia/mediawiki-services-restbase,Pchelolo/restbase,d00rman/restbase,milimetric/restbase,Pchelolo/restbase,d00rman/restbase,wikimedia/mediawiki-services-restbase,cscott/restbase,milimetric/restbase,wikimedia/restbase