Update CONTRIBUTING.md

[crockwave]

The link to the previously referenced PDF Specification was broken. Updated the link to a currently hosted version of that specification document.

What?

The link to the previously referenced PDF Specification was broken. Updated the link to a currently hosted version of that specification document.

Why?

How?

Testing?

Tested link in Preview window

New Dependencies?

Screenshots

Suggested Reading?

Anything Else?

Checklist

• I read CONTRIBUTING.md.
• I read MAINTAINERSHIP.md#pull-requests.
• I added/updated unit tests for my changes.
• I added/updated integration tests for my changes.
• I ran the integration tests.
• I tested my changes in Node, Deno, and the browser.
• I viewed documents produced with my changes in Adobe Acrobat, Foxit Reader, Firefox, and Chrome.
• I added/updated doc comments for any new/modified public APIs.
• My changes work for both new and existing PDF files.
• I ran the linter on my changes.