pdf: correct the type of some attributes

Fixed `PDFDocumentProxy.numPages` and `PDFDocumentProxy.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
  - [`.numPages`](https://github.com/mozilla/pdf.js/blob/816f2f7e1dc604d52bbde29eb9357360dbeddbbe/src/core/core.js#L476)
  - [`.fingerprint`](https://github.com/mozilla/pdf.js/blob/816f2f7e1dc604d52bbde29eb9357360dbeddbbe/src/core/core.js#L512)