Clarify surrogate key definition in readme

Old copy:

> Surrogate keys are what Fastly uses to purge indivual objects from our caches.

New copy:

> Surrogate keys are what Fastly uses to purge groups of individual objects from our caches.

This fixes the typo for "individual" and clarifies that surrogate keys are designed for batch purging, rather than purging _just_ single objects.