Update description of options keys

According to the [source](https://github.com/ardoq/analytics-clj/blob/master/src/ardoq/analytics_clj.clj#L79), the `:context` key should actually be `:options` (although `:context` would more accurately reflect the concept of a Segment [special context](https://segment.com/docs/api/tracking/track/#special-context))

Although I don't feel too strongly about the name, I figured it was best for the README to be accurate for now. Thanks for the great library!