doc: fix slirp description

net/slirp.c says:
    /* default settings according to historic slirp */
    struct in_addr net  = { .s_addr = htonl(0x0a000200) }; /* 10.0.2.0 */
    struct in_addr mask = { .s_addr = htonl(0xffffff00) }; /* 255.255.255.0 */
    struct in_addr host = { .s_addr = htonl(0x0a000202) }; /* 10.0.2.2 */
    struct in_addr dhcp = { .s_addr = htonl(0x0a00020f) }; /* 10.0.2.15 */
    struct in_addr dns  = { .s_addr = htonl(0x0a000203) }; /* 10.0.2.3 */

Which I think is not what the documentation says.
Signed-off-by: NBrad Hards <bradh@frogmouth.net>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

qemu-options.hx

@@ -1152,7 +1152,7 @@ Assign symbolic name for use in monitor commands.
 @item net=@var{addr}[/@var{mask}]
 Set IP network address the guest will see. Optionally specify the netmask,
 either in the form a.b.c.d or as number of valid top-most bits. Default is
-10.0.2.0/8.
+10.0.2.0/24.
 
 @item host=@var{addr}
 Specify the guest-visible address of the host. Default is the 2nd IP in the
@@ -1168,7 +1168,7 @@ Specifies the client hostname reported by the builtin DHCP server.
 
 @item dhcpstart=@var{addr}
 Specify the first of the 16 IPs the built-in DHCP server can assign. Default
-is the 16th to 31st IP in the guest network, i.e. x.x.x.16 to x.x.x.31.
+is the 15th to 31st IP in the guest network, i.e. x.x.x.15 to x.x.x.31.
 
 @item dns=@var{addr}
 Specify the guest-visible address of the virtual nameserver. The address must