Fixes ChannelTypeReader (#630)

src/Discord.Net.Commands/Readers/ChannelTypeReader.cs

@@ -30,7 +30,7 @@ namespace Discord.Commands
                     AddResult(results, channel as T, channel.Name == input ? 0.80f : 0.70f);
 
                 if (results.Count > 0)
-                    return TypeReaderResult.FromSuccess(results.Values);
+                    return TypeReaderResult.FromSuccess(results.Values.ToReadOnlyCollection());
             }
 
             return TypeReaderResult.FromError(CommandError.ObjectNotFound, "Channel not found.");