x86/vsyscall: Fix documentation to reflect the default mode

The default vsyscall mode has been updated from emulate to xonly for a
while. Update the kernel-parameters doc to reflect that.

Fixes: 625b7b7f ("x86/vsyscall: Change the default vsyscall mode to xonly")
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230111193211.1987047-1-sohil.mehta@intel.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -6766,11 +6766,11 @@
 			functions are at fixed addresses, they make nice
 			targets for exploits that can control RIP.
 
-			emulate     [default] Vsyscalls turn into traps and are
-			            emulated reasonably safely.  The vsyscall
-				    page is readable.
+			emulate     Vsyscalls turn into traps and are emulated
+			            reasonably safely.  The vsyscall page is
+				    readable.
 
-			xonly       Vsyscalls turn into traps and are
+			xonly       [default] Vsyscalls turn into traps and are
 			            emulated reasonably safely.  The vsyscall
 				    page is not readable.