Update docs

Author: zzooeeyy

docs/usage/oauth.md

@@ -14,7 +14,6 @@ For more information on authenticating a Shopify app please see the [Types of Au
   - [Authorization Code Grant](#authorization-code-grant)
   - [Client Credentials Grant](#client-credentials-grant)
 - [Expiring Offline Access Tokens](#expiring-offline-access-tokens)
-  - [Enabling Expiring Offline Access Tokens](#enabling-expiring-offline-access-tokens)
   - [Refreshing Access Tokens](#refreshing-access-tokens)
   - [Migrating Non-Expiring Tokens to Expiring Tokens](#migrating-non-expiring-tokens-to-expiring-tokens)
 - [Using OAuth Session to make authenticated API calls](#using-oauth-session-to-make-authenticated-api-calls)