Update PDF specification URL in CONTRIBUTING.md

[kyoungduck]

What?

Update PDF specification URL in CONTRIBUTING.md

Why?

last URL was broken.

How?

Update CONTRIBUTING.md

Testing?

No need

New Dependencies?

No

Screenshots

N/A

Suggested Reading?

Yes

Anything Else?

Checklist

• [x] I read CONTRIBUTING.md.
• [x] I read MAINTAINERSHIP.md#pull-requests.