Add grabbing password to part3c.md

When running the index.js file after adding the code to the backend the password isn't fetched from command line or anywhere else so it can't be run as is.

Author: xksteven

src/content/3/en/part3c.md

@@ -371,6 +371,8 @@ Let's get a quick start by copy-pasting the Mongoose definitions to the <i>index
 ```js
 const mongoose = require('mongoose')
 
+const password = process.argv[2]
+
 // DO NOT SAVE YOUR PASSWORD TO GITHUB!!
 const url =
   `mongodb+srv://fullstack:${password}@cluster0.o1opl.mongodb.net/?retryWrites=true&w=majority`