MAINTAINERS: repair file entry in DOCUMENTATION

Commit 1e596d5e

 ("docs: Detect variable fonts and suggest denylisting
them") adds the new script check-variable-fonts.sh and intends to refer to
it in the DOCUMENTATION section in MAINTAINERS. However, the file entry
refers to scripts/check-variable-font.sh. Note the missing "s".

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains about a
broken reference.

Repair this new file entry in the DOCUMENTATION section.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240417101429.240495-1-lukas.bulwahn@redhat.com