Use asyncio.gather instead of asyncio.wait in example.

gather will raise exception in case of problem but wait just returns done and pending lists of futures.
For getting error every future result should be retrieved, which is boring and error prone.

Doc/library/asyncio-task.rst

@@ -486,7 +486,7 @@ Example executing 3 tasks (A, B, C) in parallel::
         asyncio.ensure_future(factorial("A", 2)),
         asyncio.ensure_future(factorial("B", 3)),
         asyncio.ensure_future(factorial("C", 4))]
-    loop.run_until_complete(asyncio.wait(tasks))
+    loop.run_until_complete(asyncio.gather(*tasks))
     loop.close()
 
 Output::