support/gnuconfig: reference the correct sha1 we're using

In commit ff9f778c (support/gnuconfig: update to 2019-05-28), we
forgot to update the README to reference the sha1 we're using, keeping
the old one from 2016...

Update it now.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>