Update playhouse.rst
Hmm, but `tuple` does have `len`...

``` python
In [1]: x = (1,2,3)

In [2]: len(x)
Out[2]: 3
```
 See my sample code below. For some reason I was thinking it was just a documentation issue, but perhaps it's a bug...?  

```
mysql_db = PooledMySQLDatabase('dbname',
                               host=os.environ['DB_HOST'],
                               user=os.environ['DB_USER'],
                               passwd=os.environ['DB_PASS'],
                               max_connections=32,
                               stale_timeout=300)  # 5 minutes.

mysql_read_db = PooledMySQLDatabase('dbname',
                               host=os.environ['DB_READ_HOST'],
                               user=os.environ['DB_USER'],
                               passwd=os.environ['DB_PASS'],
                               max_connections=32,
                               stale_timeout=300)  # 5 minutes.

def _db_close(exc):
    databases = [mysql_db,mysql_read_db]
    for database in databases:
        if not database.is_closed():
            database.close() 

class BaseModel(ReadSlaveModel):
    class Meta:
        database = mysql_db
        read_slaves = [mysql_read_db] // <--- if (mysql_read_db) then i get error.                            

```
 A tuple with only one item needs a trailing comma.

(db,)

I think read_slaves needs to be a list not a tuple. I was getting `TypeError: object of type '....' has no len()` until I made this change.
