"NPM" was parsing as a relative link

I prefixed it with http://

README.md

@@ -7,7 +7,7 @@ Install it using: ```$ npm install gitbook-plugin```
 
 ## How GitBook plugin works?
 
-A plugin for GitBook is a node package that can be published on [NPM](www.npmjs.org).
+A plugin for GitBook is a node package that can be published on [NPM](http://www.npmjs.org).
 
 ### package.json