Change default no-subcommand-specified message

pluginbase-core/src/main/resources/pluginbase_en.properties

@@ -13,7 +13,7 @@ commands.missing-argument=You must specify a value for the {0}.
 commands.no-permission=You do not have permission to execute this command.
 commands.error-occurred=An error occurred while executing this command.
 commands.too-many-arguments=Too many arguments! Correct usage: /{0}
-commands.no-subcommand-specified=You must specify a subcommand.
+commands.no-subcommand-specified=You must specify a valid subcommand.
 commands.on-cooldown=You must wait {0} before using this command again.
 commands.number-not-in-range={0} must be between {1} and {2} (found {3}).
 commands.must-be-player=You must be a player to use this command.