qemu-doc: Fix "-net van" typo

While Andrew S. Tanenbaum has a point by saying "Never underestimate the
bandwidth of a station wagon full of tapes hurtling down the highway",
we don't support that way of transportation in QEMU yet, so replace the
typo with the correct word "vlan".
Signed-off-by: NThomas Huth <thuth@redhat.com>
Message-Id: <1502365466-19432-1-git-send-email-thuth@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qemu-doc.texi

@@ -3243,7 +3243,7 @@ the ``-netdev user,guestfwd=ARGS'' argument instead.
 
 @subsection -net vlan (since 2.9.0)
 
-The ``-net van=NN'' argument is partially replaced with the
+The ``-net vlan=NN'' argument is partially replaced with the
 new ``-netdev'' argument. The remaining use cases will no
 longer be directly supported in QEMU.