x86/entry: spell EBX register correctly in documentation

As EBS does not mean anything reasonable in the context it is used, it
seems like a misspelling for EBX.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>