Documentation/ABI/: update version number references from 2.6.x to 3.x

Update the version number references from 2.6.x to 3.x
Also correct spelling of "May" (was spelled "Mai").

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>