database.url syntax clarity

[LeviSnoot]

This might help inexperienced people like me when setting up zipline.

In the Database Config section of the docs, examples are given for various database urls. To me it was confusing to see user, password and database all being referred to as 'zipline'. I figured it out with some trial and error but could've saved some time at this step.

Original:

// for postgresql
'postgres://zipline:zipline@localhost:5432/zipline'

// for mysql
'mysql://zipline:zipline@localhost:3306/zipline'

// for sqlite
'file:zipline.db'

My suggestion:

// for postgresql
'postgres://user:password@localhost:5432/database'

// for mysql
'mysql://user:password@localhost:3306/database'

// for sqlite
'file:zipline.db'