Update ACL documentation to refer to new property name

[maartenkamoen]

The documentation at https://docs.siderolabs.com/omni/security-and-authentication/using-saml-with-omni/auto-assign-roles-to-saml-users refers to deprecated property assignroleonregistration. This should be adjusted to assignrole as indicated by this error:

$ omnictl apply -f samllabelrules.yaml
Error: failed to apply resources from file "samllabelrules.yaml": failed to update resource 'assign-admin-to-admin-role' 'SAMLLabelRules.omni.sidero.dev': failed to update resource 'assign-admin-to-admin-role' 'SAMLLabelRules.omni.sidero.dev': rpc error: code = InvalidArgument desc = failed to validate: 1 error occurred:
        * assignroleonregistration is deprecated, please use assignrole instead