[5.1] webauthn missing string

Adds a missing string. As its an error message string and virtually impossible to reach then code review should be enough.

administrator/language/en-GB/plg_system_webauthn.ini

@@ -6,6 +6,7 @@
 PLG_SYSTEM_WEBAUTHN="System - Passkey (Passwordless) Login"
 PLG_SYSTEM_WEBAUTHN_CANNOT_ADD_FOR_A_USER="You cannot add or remove passkeys on behalf of users. Users must login, and set up their own devices."
 PLG_SYSTEM_WEBAUTHN_DESCRIPTION="Enables passwordless authentication using passkeys. Please note that the passkeys tab in the user profile editor and the passkeys login buttons will only be displayed if the user is accessing the site over HTTPS. Registering passkeys and using them to log into your site will only work when your site is using a valid certificate, signed by a Certificate Authority the user's browser trusts."
+PLG_SYSTEM_WEBAUTHN_ERR_AJAX_INVALIDACTION="Invalid ajax action"
 PLG_SYSTEM_WEBAUTHN_ERR_CANNOT_FIND_USERNAME="Cannot find the username field in the login module. Sorry, Passwordless authentication will not work on this site unless you use a different login module."
 PLG_SYSTEM_WEBAUTHN_ERR_CANT_STORE_FOR_GUEST="Cannot possibly store credentials for Guest user!"
 PLG_SYSTEM_WEBAUTHN_ERR_CORRUPT_STORED_CREDENTIAL="The stored credentials are corrupt for your user account. Log in using another method, then remove and add again your passkey."