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

In commit ff9f778c66f4 (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>
(cherry picked from commit 3bf545da78b831339f5f717dfb4d120d46ecd929)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/gnuconfig/README.buildroot

@@ -24,4 +24,4 @@ Currently no patches are needed, but they may be needed again in the
 future.
 
 The current Buildroot version is based on the Git commit
-dafd8e767ec87b90aac62f0fcedd11944c84b50a of the config.git repository.
+104ee6463c4bfaac3f3029d9be9bdd6e93879323 of the config.git repository.