Correct the link to pdf.js library on project page

Problem/Motivation

The link to the Mozilla pdf.js library on the project page is incorrect. The correct link where the library can be downloaded is https://mozilla.github.io/pdf.js/getting_started/#download

The project is also available on Github at https://github.com/mozilla/pdf.js

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes