trace: update old trace events in docs

Commit c5f1ad429cdf26023cf331075a7d327708e3db6d ("block: Remove
bdrv_aio_readv/writev/flush()") removed
bdrv_aio_readv()/bdrv_aio_writev() so the example in the tracing
documentation is no longer valid.

Reported-by: Wang Dong <dongdwdw@linux.vnet.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170714133111.27359-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
