Replace dash with underscore in Python lib docs.

Greetings, I've fixed dash(`-`) of `time-precision` to underscore(`_`) in Python lib docs so that word is `time_precision` now since dash in argument/variable name is invalid in Python. ([Preview](https://github.com/yoloseem/influxdb.org/blob/patch-1/source/docs/v0.8/client_libraries/python.md#write_points_with_precisiondata-time_precisions))