mediawiki.api.watch: Fix promise return value format

Follow-up to Ic77b546c.

Bug: 62422
Change-Id: I2ac9e0da0f1c825e1cefd98d21406fcb089c0827
(cherry picked from commit 3fa6c0f3d10f76b17dc9fcb115e8013e1a57985a)
Merge "mediawiki.api.watch: Fix promise return value format" into wmf/1.23wmf17