Fix environment list in plugin load order example

Corrects the documentation to state that the plugin will load in 'development', 'test', and 'myCustomEnv' environments, matching the provided code example.

Author: jamesfredley

grails-doc/src/en/guide/plugins/understandingPluginLoadOrder.adoc

@@ -101,7 +101,7 @@ It's not only plugin load order that you can control. You can also specify which
 def environments = ['development', 'test', 'myCustomEnv']
 ----
 
-In this example, the plugin will only load in the 'development' and 'test' environments.
+In this example, the plugin will only load in the 'development', 'test' and 'myCustomEnv' environments.
 
 The properties can be one of: