dpif-netdev-unixctl: Change 'masked' to 'megaflow'.

In the review process of the original patch 'masked hits' stat
was renamed to 'megaflow hits', but the man page wasn't updated.

Fixes: 6553d06 ("dpif-netdev: Add dpif-netdev/pmd-stats-*
                      appctl commands.")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>

lib/dpif-netdev-unixctl.man

@@ -11,7 +11,7 @@ Shows performance statistics for one or all pmd threads of the datapath
 \fIdp\fR. The special thread "main" sums up the statistics of every non pmd
 thread.
 
-The sum of "emc hits", "masked hits" and "miss" is the number of
+The sum of "emc hits", "megaflow hits" and "miss" is the number of
 packet lookups performed by the datapath. Beware that a recirculated packet
 experiences one additional lookup per recirculation, so there may be
 more lookups than forwarded packets in the datapath.