Document error about `get_n_pages`

[NewUserHa]

https://libvips.github.io/pyvips/vimage.html#pyvips.Image.get_n_pages

get_n_pages actually returns number of pages of the file, and it's not the page number of pages in the file the image was loaded.

Also the libvips document says:
https://www.libvips.org/API/8.17/method.Image.get_n_pages.html

[−]
Description
[src]
Fetch and sanity-check VIPS_META_N_PAGES. Default to 1 if not present or crazy.

This is the number of pages in the image file, not the number of pages that have been loaded into image.

[−]
Return value
Type: int

The number of pages in the image file.

The error should be fixed.
Should create a PR?

[jcupitt]

Hi @NewUserHa, ooops!

You're right, please make a PR.