Link JavaScript to HTML Code for Simple Calculator

We need to properly link the JavaScript file to the HTML file for our Efaculator project. This will ensure that the calculator's functionality is enabled and working correctly when the HTML file is opened in a browser.

**Tasks:**
1. Create or verify the existence of the JavaScript file:
Ensure that there is a calculator.js file (or an appropriately named JavaScript file) containing the calculator's logic.
2. Link the JavaScript file in the HTML:
Add a tag in the HTML file (index.html) to link the JavaScript file.
Ensure the script tag is placed at the correct location (usually at the end of the body tag) for optimal loading.