changed install command for NPM

xmldom is no longer published on NPM, but now published from their github repo.

see their issue for more info here:
xmldom/xmldom#271

README.md

@@ -17,7 +17,7 @@ Install with [npm](http://github.com/isaacs/npm):
 
 xpath is xml engine agnostic but we recommend [xmldom](https://github.com/xmldom/xmldom):
 
-    npm install xmldom
+    npm install @xmldom/xmldom
 
 ## API Documentation
 
@@ -130,4 +130,4 @@ console.log(author)
 [MDN]: https://developer.mozilla.org/en/docs/Web/API/Document/evaluate
 
 ## License
-MIT
+MIT