Incorrect PRECISION_NANOSECONDS marker

[bendoh]

In https://github.com/influxdata/influxdb-php/blob/master/src/InfluxDB/Database.php#L35

    const PRECISION_NANOSECONDS = 'n';

Technically this is incorrect as the parameter should be 'ns' not 'n', but since the default is nanoseconds anyway, it's basically just a red herring that does nothing. See https://archive.docs.influxdata.com/influxdb/v1.2/tools/api/#write — The "fix", if any, is to just change precision=n to precision=ns, or drop precision altogether and just assume nanoseconds throughout.