Bug#817999: apt: fix message in spanish localization

[Camaleón <noelamac@gmail.com>]

Hi!,

I'm a member of the Spanish translation team. The given diff cannot be 
applied "as is" without previous review by our team and also the 
translator of that file (CC'ing him now). 

A more appropriate translation would be:

#: cmdline/apt-mark.cc
#, c-format
msgid "%s can not be marked as it is not installed.\n"
msgstr "%s no se puede marcar porque no está instalado.\n"

But the final decision is on the owner the file.

Thanks.

-- 
Camaleón