Set touch policy to fixed.

Setting the touch policy to `on` does not prevent the policy from
later being turned off again. Setting it to `fixed` is more secure
because it can not be turned off.

If someone wants to disable the touch policy they can always restore
the keys from the backups created in the guide.

README.md

@@ -2256,19 +2256,19 @@ To require a touch for each key operation, install [YubiKey Manager](https://dev
 Authentication:
 
 ```console
-$ ykman openpgp set-touch aut on
+$ ykman openpgp set-touch aut fixed
 ```
 
 Signing:
 
 ```console
-$ ykman openpgp set-touch sig on
+$ ykman openpgp set-touch sig fixed
 ```
 
 Encryption:
 
 ```console
-$ ykman openpgp set-touch enc on
+$ ykman openpgp set-touch enc fixed
 ```
 
 YubiKey will blink when it is waiting for a touch. On Linux you can also use [yubikey-touch-detector](https://github.com/maximbaz/yubikey-touch-detector) to have an indicator or notification that YubiKey is waiting for a touch.