docs/manual: contribute: fix license string for Buildroot license

As per section 12.2 "Complying with the Buildroot license", Buildroot
is released under GPL version 2 or (at your option) any later version.
But SPDX license string for Buildroot license in section 21.5
"Submitting patches" refers to only GPL version 2. Fix this
inconsistency by using correct SPDX identifier.

Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>