docs: Ensure accessTokenExpiresAt is required

See [comment here](#490 (comment)) indicating that the documentation is incorrect; This simply brings the documentation in line with the implementation.

Author: Matt Grande

docs/model/spec.rst

@@ -195,7 +195,7 @@ An ``Object`` representing the access token and associated data.
 +------------------------------+--------+--------------------------------------------------+
 | token.accessToken            | String | The access token passed to ``getAccessToken()``. |
 +------------------------------+--------+--------------------------------------------------+
-| [token.accessTokenExpiresAt] | Date   | The expiry time of the access token.             |
+| token.accessTokenExpiresAt   | Date   | The expiry time of the access token.             |
 +------------------------------+--------+--------------------------------------------------+
 | [token.scope]                | String | The authorized scope of the access token.        |
 +------------------------------+--------+--------------------------------------------------+