docs: fix npm docs link (#1324)

Author: arvindell

packages/mail/README.md

@@ -30,7 +30,7 @@ source ./sendgrid.env
 ```
 
 ## Install Package
-The following recommended installation requires [npm](https://npmjs.org/). If you are unfamiliar with npm, see the [npm docs](https://npmjs.org/doc/). Npm comes installed with Node.js since node version 0.8.x, therefore, you likely already have it.
+The following recommended installation requires [npm](https://npmjs.org/). If you are unfamiliar with npm, see the [npm docs](https://docs.npmjs.com/). Npm comes installed with Node.js since node version 0.8.x, therefore, you likely already have it.
 
 ```sh
 npm install --save @sendgrid/mail