update string parsing example in README

- replace `document inspect` with `document root nodes inspect`
- update associated screenshot

Author: capsulecorplab

README.md

@@ -54,7 +54,7 @@ xmlString := '<?xml version="1.0" encoding="UTF-8"?>
   <country code="dz" handle="algeria" continent="africa" iso="12">Algeria</country>
 </countries>'.
 
-(XMLDOMParser parse: xmlString) document inspect 
+(XMLDOMParser parse: xmlString) document root nodes inspect
 ```
 
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