fix mdn link (#51)

Author: drillprop

src/commands/mdn.ts

@@ -18,7 +18,7 @@ const mdn: Command = {
     const firstDocument = data.documents[0];
     return msg.channel.send([
       firstDocument.excerpt,
-      `https://developer.mozilla.org/${firstDocument.slug}`,
+      `https://developer.mozilla.org/en-US/docs/${firstDocument.slug}`,
     ]);
   },
 };