docs: Update code example of prefetching related objects

[jrycw]

To ensure equivalence between the two code snippets, I believe we may be missing the .first() method call in the last one.

# These are equivalent:
ticket = await Ticket.objects(
    Ticket.concert.all_related()
).first()

ticket = await Ticket.objects().prefetch(
    Ticket.concert.all_related()
) # <= missing `.first()` here

[dantownsend]

@jrycw You're right - great catch! 👍