Make python examples PEP8 compliant.

These are my proposed fixes for rethinkdb/rethinkdb#2534.
In this example (and a few others), neither the old or the new syntax are very good at representing the query. Of all the ways I've seen ReQL being formatted, I like this one best:

```
(r.table("authors")
  .filter(r.row['name'] == "William Adama")
  .update({"rank": "Admiral"})
  .run())
```

Perhaps with parentheses on separate lines.