Update novice R lesson 01-starting-with-data

Per a conversation with @jdblischak I have removed the section in the first lesson that used `seq()` to index a data.frame. 

This was originally included to mirror the first python lesson which used strides (see #692)  That has been removed and this simply keeps that first novice R lesson in-line with that change.  

Both the .Rmd and .md files have been updated.