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>

arch/x86/entry/entry_64.S

@@ -1058,7 +1058,7 @@ END(error_entry)
 
 
 /*
- * On entry, EBS is a "return to kernel mode" flag:
+ * On entry, EBX is a "return to kernel mode" flag:
  *   1: already in kernel mode, don't need SWAPGS
  *   0: user gsbase is loaded, we need SWAPGS and standard preparation for return to usermode
  */