Fix qemu -net user,hostfwd= example

Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>
(cherry picked from commit aa375206)

qemu-options.hx

@@ -968,7 +968,7 @@ the guest, use the following:
 
 @example
 # on the host
-qemu -net user,hostfwd=tcp:5555::23 [...]
+qemu -net user,hostfwd=tcp::5555-:23 [...]
 telnet localhost 5555
 @end example