In current http.md, the valid field's description is
valid: a boolean flag indicating whether the lane is a valid choice in the current maneuver.
But the example is string rather than bool:

{
    "indications": ["left", "straight"],
    "valid": "false"
}

By trying real case, the value is bool in json response.
http://router.project-osrm.org/route/v1/driving/-121.685212%2C37.233934%3B-121.565400%2C37.024500%3B-121.380400%2C36.989900%3B-121.017600%2C37.109400%3B-120.237900%2C36.254100%3B-119.331600%2C35.354600%3B-118.174600%2C35.068600%3B-116.817627%2C34.912962?overview=false&steps=true

"lanes": [
{
"valid": true,
"indications": [
"left"
]
},

So I think it's just the doc example error. Will create a PR to correct it.