Update the npm command

updated the npm command to execute the correct command

Author: rkrevolution

CONTRIBUTING.md

@@ -59,7 +59,7 @@ When you're done making changes:
 
 ```bash
 # Rebuild HTML content
-npm run book:build
+ npm run test:404
 
 # Check for 404s
 npm run test:404 # See notes below about this