Update sequence usage example in the introduction
Update sequence usage example in the introduction

Since commit 13d310fa14f4e4b9a559f8b7887f2a2492357013, sequences have
started on 0 by default instead of 1. Update the usage example in the
introduction text to reflect that.

FactoryBoy/factory_boy,rrauenza/factory_boy,rbarrois/factory_boy