Set max width for page numbers in index

The table of contents can have text as page numbers. If the text
is text for the page is too long, it does not allow to see the 
table of contents.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=660113



Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>