dpkg: Clarify --update-avail usage error message

Unify the reference to the Packages-file to match the one used on the
man pages and on the --help output.

Closes: #628055

debian/changelog

@@ -192,6 +192,7 @@ dpkg (1.16.1) UNRELEASED; urgency=low
     to open the to be installed paths for writing.
     Thanks to Jonathan Nieder <jrnieder@gmail.com>. Closes: #635683
   * Clarify the default dpkg-deb compression-levels on the man page.
+  * Clarify dpkg --update-avail usage error message. Closes: #628055
 
   [ Updated dpkg translations ]
   * German (Sven Joachim). Closes: #620312

src/update.c

@@ -49,7 +49,8 @@ updateavailable(const char *const *argv)
     break;
   case act_avreplace: case act_avmerge:
     if (!sourcefile || argv[1])
-      badusage(_("--%s needs exactly one Packages file argument"),cipaction->olong);
+      badusage(_("--%s needs exactly one Packages-file argument"),
+               cipaction->olong);
     break;
   default:
     internerr("unknown action '%d'", cipaction->arg_int);