Update dockblock for AbstractProvider::prepareAccessTokenResponse (#1025

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This commit updates the docblock to reflect the expected type for $result.

src/Provider/AbstractProvider.php

@@ -815,7 +815,7 @@ abstract protected function checkResponse(ResponseInterface $response, $data);
      * Custom mapping of expiration, etc should be done here. Always call the
      * parent method when overloading this method.
      *
-     * @param  mixed $result
+     * @param  array<string, mixed> $result
      * @return array
      */
     protected function prepareAccessTokenResponse(array $result)