Spanish apt-mark translation fix

"%s can not be marked as it is not installed." was incorrectly
translated as "%s no se puede marcar como no instalado.\n",
which means "%s can not be marked as not installed."

Thanks to Marcos Del Sol Vives for reporting & to the spanish
translation team – and in particular Camaleón and Venturi –
for review and correction of this issue!

[commit message by commiter]

Closes: 817999

po/es.po

@@ -1397,7 +1397,7 @@ msgstr "detecta el proxy usando apt.conf"
 #: cmdline/apt-mark.cc
 #, c-format
 msgid "%s can not be marked as it is not installed.\n"
-msgstr "%s no se puede marcar como no instalado.\n"
+msgstr "%s no se puede marcar porque no está instalado.\n"
 
 #: cmdline/apt-mark.cc
 #, c-format