fix preferred content check for 1 semitrusted or better copy

Let's make semitrusted+:1 mean that, since it cannot be expressed easily
with the current syntax (could use (semitrusted:1 or trusted:1), but that
does not scale to higher values than 2 copy, and also fails if I add more
trust levels).

Thanks to Andy for spotting this bug by just reading my blog.

Types/StandardGroups.hs

@@ -74,4 +74,4 @@ preferredContent UnwantedGroup = "exclude=*"
 {- Most repositories want any content that is only on untrusted
  - or dead repositories. -}
 lastResort :: String -> String
-lastResort s = "(" ++ s ++ ") or (not copies=semitrusted:1)"
+lastResort s = "(" ++ s ++ ") or (not copies=semitrusted+:1)"