Invalid server name env variable key

[3umaGH]

Wiki page to modify:
Bukkit-Configuration.md
Bungee-Configuration.md

Suggested modifications

Both contain invalid environment variable for server name.

Proposed changes:

- Plugin.ServerName -> PLAN_PLUGIN_SERVERNAME
+ Server.ServerName -> PLAN_SERVER_SERVERNAME

Additional context

[AuroraLS3]

Fixed