gnat in erl_driver documentation

[Vance Shipley]

~/doc/html/erl_driver.html:

In the description of the driver_async function it refers to the
callback in the driver entry as both "ready_async" and "async_ready":

   When the async operation is done, ready_async driver entry function
   is called. If async_ready is null in the driver entry, the async_free
   function is called  instead.

The correct entry is "ready_async".

	-Vance