Outdated time_precision docstring in line_protocol.py

[chrominium97]

• InfluxDB version: 1.8.0
• InfluxDB-python version: 5.3.0
• Python version: e.g. 3.8.3
• Operating system version: macOS 10.15.6

The docstrings for methods write_points and send_packet do not state 'n' nor 'h' as possible values, although it is currently supported.