wrong set_nquads example

[eyalsus]

Took me a while to figure out how to use set_nquads.
I notice that the README.md example is wrong:

# If you want to use N-Quads, use this instead:
# txn.mutate(set_nquads='_:alice "Alice"')

should be:

# txn.mutate(set_nquads='_:alice "Alice" .')