Fix 'Omibus' spelling in left nav
Not a new page; a spelling fix. Omibus
→ Omnibus
As this doesn't change the destination of a link in the left nav, just the spelling of the displayed text, I think this merge request is safe to merge in the 17th-22nd window. I've cc'ed @eread as the release post person for this month, in case I've made a Terrible Mistake™.