docs: replace links pointing to master with links pointing to main (#463)

Author: kevinpagtakhan

README.md

@@ -18,11 +18,11 @@ A JavaScript SDK for tracking events and revenue to [Amplitude](https://www.ampl
 
 ## Installation and Quick Start
 * For using the SDK, please visit our :100:[Developer Center](https://developers.amplitude.com/docs/javascript).
-* For developing the SDK, please visit our [CONTRIBUTING.md](https://github.com/amplitude/Amplitude-JavaScript/blob/master/CONTRIBUTING.md).
+* For developing the SDK, please visit our [CONTRIBUTING.md](https://github.com/amplitude/Amplitude-JavaScript/blob/main/CONTRIBUTING.md).
 
 ## Demo Pages
-* A [demo page](https://github.com/amplitude/Amplitude-JavaScript/blob/master/test/browser/amplitudejs.html) showing a simple integration on a web page.
-* A [demo page](https://github.com/amplitude/Amplitude-JavaScript/blob/master/test/browser/amplitudejs-requirejs.html) showing an integration using RequireJS.
+* A [demo page](https://github.com/amplitude/Amplitude-JavaScript/blob/main/test/browser/amplitudejs.html) showing a simple integration on a web page.
+* A [demo page](https://github.com/amplitude/Amplitude-JavaScript/blob/main/test/browser/amplitudejs-requirejs.html) showing an integration using RequireJS.
 * A [demo page](https://github.com/amplitude/GTM-Web-Demo) demonstrating a potential integration with Google Tag Manager.