(#853): Fix incorrect suggestion in API docs about measuring geometries
(#853): Fix incorrect suggestion in API docs about measuring geometries

It suggested get_geometry_for_layer instead of
get_geometry_for_element; the latter is correct.

Fixes https://gitlab.gnome.org/GNOME/librsvg/-/issues/853

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/682>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg