Replace references to fabric in purge cache docs
Replace references to fabric in purge cache docs

[Trello](https://trello.com/c/AGdifoDb/250-remove-fabric-and-replace-references-in-docs)

We have deprecated fab and are removing references to it from all our
dev docs.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs