ethdev: fix function name in comment

[ upstream commit 4b98bef701c0bf39752031dd8943326945e2b5d1 ]

For those using the function comments as a guide, provide the name of
the correct callback function to use when wanting to count dropped
packets from the ethdev Tx buffering system.

Fixes: d6c99e6 ("ethdev: add buffered Tx")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@amd.com>
Acked-by: Huisong Li <lihuisong@huawei.com>

lib/librte_ethdev/rte_ethdev.h

@@ -3325,7 +3325,7 @@ rte_eth_tx_buffer_init(struct rte_eth_dev_tx_buffer *buffer, uint16_t size);
  * for example, to count dropped packets, or to retry transmission of packets
  * which cannot be sent, this function should be used to register a suitable
  * callback function to implement the desired behaviour.
- * The example callback "rte_eth_count_unsent_packet_callback()" is also
+ * The example callback "rte_eth_tx_buffer_count_callback()" is also
  * provided as reference.
  *
  * @param buffer