update SerializerMethodField example in docs

[afolksetapart]

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Description

This PR will update the code in the example below (as seen in the docs), which will throw the following error if run:

AssertionError: ("Creating a ModelSerializer without either the 'fields' attribute or the 'exclude' attribute has been deprecated since 3.3.0, and is now disallowed. Add an explicit fields = 'all' to the AlbumSerializer serializer.",)

from django.contrib.auth.models import User
from django.utils.timezone import now
from rest_framework import serializers

class UserSerializer(serializers.ModelSerializer):
    days_since_joined = serializers.SerializerMethodField()

    class Meta:
        model = User

    def get_days_since_joined(self, obj):
        return (now() - obj.date_joined).days 

This PR adds the required fields = '__all__' to the example and may help folks following along for the first time.

[lovelydinosaur]

Fantastic, thanks!

[afolksetapart]

Thanks @tomchristie !