Correct URL for NPM URL reference

URL for commented reference has been updated

GitBook.gitignore

@@ -4,7 +4,7 @@
 
 ## Dependency directory
 ## Commenting this out is preferred by some people, see
-## https://npmjs.org/doc/faq.html#Should-I-check-my-node_modules-folder-into-git
+## https://docs.npmjs.com/misc/faq#should-i-check-my-node_modules-folder-into-git
 node_modules
 
 # Book build output