Added System.Linq reference (#1470)

Use of IReadOnlyCollection#Any requires a reference to System.Linq. It's a small thing, but better copy-paste-ability is never a bad thing.

docs/guides/commands/samples/preconditions/require_role.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 using System.Threading.Tasks;
 using Discord.Commands;
 using Discord.WebSocket;