README: Increase the underline length
README: Increase the underline length

Currently the underline is shorter than the "SWU image signing"
subtitle.

Make the underline fill the whole "SWU image signing" subtitle
for better readability.

Signed-off-by: Fabio Estevam <288a049a20dc4091188b72022d411ad1375e6737@gmail.com>

sbabic/meta-swupdate,sbabic/meta-swupdate