docs: fix anonymise_<field_name> example in usage docs

[kevinmarsh]

I noticed the example in the usage docs doesn't match the privacy meta docs says:

django-gdpr-assist/docs/privacy_meta.rst

Lines 100 to 110 in 250c63f

	``anonymise_<field_name>(self, instance)``
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Custom function to anonymise the named field, for when the standard anonymisers
	won't produce the desired result. This should also be used for custom field
	types.
	Field name must appear in the ``fields`` list.
	It should not return a value; instead it should operate directly on the
	instance.