doc: fix maximum packet size of virtio driver

[ upstream commit c4c1b97 ]

According to VIRTIO_MAX_RX_PKTLEN macro definition, for virtio driver
currently supported pkt size is 9728.

Fixes: fc1f275 ("doc: programmers guide")

Signed-off-by: Yi Li <liyi1@chinatelecom.cn>

doc/guides/nics/virtio.rst

@@ -43,7 +43,7 @@ Features and Limitations of virtio PMD
 In this release, the virtio PMD provides the basic functionality of packet reception and transmission.
 
 *   It supports merge-able buffers per packet when receiving packets and scattered buffer per packet
-    when transmitting packets. The packet size supported is from 64 to 1518.
+    when transmitting packets. The packet size supported is from 64 to 9728.
 
 *   It supports multicast packets and promiscuous mode.