Document: fix doc of get_n_pages() to keep correct with libvips document (#567)

* Update vimage.py

* Update vimage.py

* Update vimage.py

Author: NewUserHa

pyvips/vimage.py

@@ -1673,8 +1673,8 @@ def atan2(self, other):
     def get_n_pages(self):
         """Get the number of pages in an image file, or 1.
 
-        This is the number of pages in the file the image was loaded from, not
-        the number of pages in the image.
+        This is the number of pages in the image file, not the number of pages
+        that have been loaded into image.
 
         To get the number of pages in an image, divide the image height by
         the page height.