i40e: cleanup wording in a header comment

Fix up the English in the header comment for i40e_ptp_tx_hang.
Signed-off-by: NJacob Keller <jacob.e.keller@intel.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40e/i40e_ptp.c

@@ -317,7 +317,7 @@ void i40e_ptp_rx_hang(struct i40e_pf *pf)
  * This watchdog task is run periodically to make sure that we clear the Tx
  * timestamp logic if we don't obtain a timestamp in a reasonable amount of
  * time. It is unexpected in the normal case but if it occurs it results in
- * permanently prevent timestamps of future packets
+ * permanently preventing timestamps of future packets.
  **/
 void i40e_ptp_tx_hang(struct i40e_pf *pf)
 {