docs: net: ena: Fix ena_start_xmit() function name typo

The ena.rst documentation referred to end_start_xmit() when it should refer
to ena_start_xmit(). Fix the typo.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Acked-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>