jsr223.md: add example for scriptExtension.get() (#2418)

Related to openhab/openhab-core#4368

configuration/jsr223.md

@@ -237,6 +237,7 @@ Note that prior to openHAB 3, script ordering was performed alphanumerically bas
 To facilitate JSR223 scripting, several openHAB-related variables are automatically predefined within `ScriptExtension` presets.
 They can be loaded into the script context using `scriptExtension.importPreset(String preset)`, e.g. `scriptExtension.importPreset("RuleSimple")`.
 The `default` preset is preloaded, so it does not require importing.
+With `scriptExtension.get("automationManager")` the `automationManager` can be made available without loading a preset.
 
 - [Overview](#overview)
   - [Example rules for a first impression](#example-rules-for-a-first-impression)