Fix prefix command content parse

bot/src/main.ts

@@ -30,7 +30,7 @@ client.bot.on("message", async m => {
         return;
     }
 
-    if (m.content.startsWith(COMMAND_PREFIX) && await plgMgr.runCommand("prefix", m, m.content)) {
+    if (m.content.startsWith(COMMAND_PREFIX) && await plgMgr.runCommand("prefix", m, m.content.substring(COMMAND_PREFIX.length))) {
         return;
     }