Added notice about placeholders with arena ids _

Author: Tigerpanzer02

docs/minigame/placeholders.mdx

@@ -14,6 +14,10 @@ If you got [**PlaceholderAPI**](https://www.spigotmc.org/resources/6245/) instal
 - ... use the [**placeholders**](https://www.spigotmc.org/wiki/placeholderapi-placeholders/) from [**PlaceholderAPI**](https://www.spigotmc.org/resources/6245/) on our plugin.
 - ... 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
+:::
+ 
 :::info
 For our *PlaceholderAPI placeholders* you will need to replace...
 
@@ -211,4 +215,4 @@ You will need to replace...
 | `%arena_option_reset_blocks_in%`      | `%thebridge_arena_ARENAID_option_reset_blocks_in%`      | Get the count of block reset          | ARENA |
 
 </TabItem>
-</Tabs>
+</Tabs>