chore: add command description for mc login command

src/main/kotlin/cn/rtast/fancybot/commands/lookup/MinecraftCommand.kt

@@ -389,6 +389,7 @@ class MCPingCommand : BaseCommand() {
     }
 }
 
+@CommandDescription("获取MC账号的AccessToken")
 class MCLoginCommand : BaseCommand() {
     override val commandNames = listOf("/mclogin")