Update placeholders.mdx

Author: Tigerpanzer02

docs/minigame/placeholders.mdx

@@ -15,7 +15,7 @@ If you got [**PlaceholderAPI**](https://www.spigotmc.org/resources/6245/) instal
 - ... use our placeholders in other plugins.
 
 :::warning
-If you got a arena id which contains "_" rename the arena id to a name without "_" in your arenas.yml
+If you got a arena id which contains "\_" rename the arena id to a name without "\_" in your arenas.yml
 :::
 
 :::info