Correct the type of PDFDocumentProxy.numPages and .fingerprint

The type of them is a property, not a function or a method.

Reference:
- API spec: https://github.com/mozilla/pdf.js/blob/305274cd45ed3b4931f983b31ebd4ffb999fb4c6/test/unit/api_spec.js#L45-L50
- Impl: https://github.com/mozilla/pdf.js/blob/816f2f7e1dc604d52bbde29eb9357360dbeddbbe/src/core/core.js#L476 and #L512
Merge pull request #2043 from Kroisse/pdf.js

pdf: correct the type of some attributes