Documentation/networking: fix default_ttl typo in mpls-sysctl

default_ttl should be integer instead of bool

Reported-by: Ying Xu <yinxu@redhat.com>
Fixes: a59166e4 ("mpls: allow TTL propagation from IP packets to be configured")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>