configs:nitrogen{6sx, 6x, 7, 8m}: fix typo in kernel headers version

A typo has been introduced during the last version bump [1].

[1] 00252b10

Fixes:
[nitrogen6sx]
https://gitlab.com/buildroot.org/buildroot/-/jobs/454255632
[nitrogen6x]
https://gitlab.com/buildroot.org/buildroot/-/jobs/454255635
[nitrogen7]
https://gitlab.com/buildroot.org/buildroot/-/jobs/454255638
[nitrogen6m8]
https://gitlab.com/buildroot.org/buildroot/-/jobs/454255640



Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Gary Bisson <bisson.gary@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>