Update nxml-mode.texi: completion now gives xmlns="-!-"

* doc/misc/nxml-mode.texi (Completion): As of 2016-01-16 "* lisp/nxml:
Use standard completion; it also works for company-mode", completing
an attribute when there is only one candidate inserts both quotes.
Update the example accordingly.
1 job for emacs-26 in 28 minutes and 35 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #1305
test

00:28:35