Docs: clarify where counter variables start
Docs: clarify where counter variables start

The way the guide is written now, it's not clear if the first counter value will be `0` or `1`, so I made it explicit that it's `0`.

Co-authored-by: Petrik de Heus <b32f3936c414798c131b08482ad06aae6b98a396@deheus.net>

yawboakye/rails,yawboakye/rails,esparta/rails,rails/rails,rails/rails,rails/rails,MSP-Greg/rails,BlakeWilliams/rails,esparta/rails,kmcphillips/rails,esparta/rails,georgeclaghorn/rails,georgeclaghorn/rails,kmcphillips/rails,BlakeWilliams/rails,MSP-Greg/rails,kmcphillips/rails,yawboakye/rails,Erol/rails,Erol/rails,BlakeWilliams/rails,georgeclaghorn/rails,MSP-Greg/rails,yahonda/rails,georgeclaghorn/rails,yawboakye/rails,BlakeWilliams/rails,Erol/rails,Erol/rails,esparta/rails,MSP-Greg/rails,yahonda/rails,kmcphillips/rails,rails/rails,yahonda/rails,yahonda/rails