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 <festevam@gmail.com>

README

@@ -44,7 +44,7 @@ or
 `@SWU_AUTO_VERSION:<package-name>@<package-data-variable>`
 
 SWU image signing
-------------
+-----------------
 
 There are 3 signing mechanisms supported by meta-swupdate at the moment: