Do not mis-spell the `gitignore` manual page link

Even in translations, links to the overview pages (such as the one
describing the `.gitignore` file) need to refer to the actual correct
pages whose names are not to be translated.

This was reported by Todd Zullinger as a broken link to the manual page
in https://lore.kernel.org/git/ZWVIRXodL9pQZbtj@pobox.com/

.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>