bug#6776: 24.0.50; nxml mode binds completion to c-return

[Chong Yidong]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> !   (add-hook 'completion-at-point-functions  'nxml-complete nil 'local)
>
> No: completion-at-point-functions should be functions that return
> completion data (that is then passed to the standard completion UI).
> Check C-h v completion-at-point-functions.

I've committed a corrected fix.