ElastiCache::SubnetGroup SubnetIds Property should be type of list<string>

[jimleuk]

Actual:

"AWS::ElastiCache::SubnetGroup": {
    ...
    "SubnetIds": {
        "$ref": "basic_types.json#/definitions/string"
    }
    ...
}

Expected:

"AWS::ElastiCache::SubnetGroup": {
    ...
    "SubnetIds": {
        "$ref": "basic_types.json#/definitions/list<string>"
    }
    ...
}`