Update arg labels for send, subscribe

[gordonbrander]

Fixes #7

Per https://www.swift.org/documentation/api-design-guidelines/#argument-labels,

If the first argument forms part of a grammatical phrase, omit its label.

This is the case for store.send(_), so we remove the label.

When the first argument forms part of a prepositional phrase, give it an argument label.

We subscribe to a publisher, therefore store.subscribe(to:)