updating update_items.py

[xlt208]

This PR updates update_items.py, which is for syncing notebook samples from this repo to ArcGIS Online. The script needs to be updated so it can be run to address #1520.

Changes:

1. Added a cmd argument to pass in a notebook runtime version if needed
2. Added logic to get the latest notebook runtime version by default
3. Updated syntax for sharing item
4. Specified encoding for open()
5. Fixed typo and adjusted formatting based on suggestions from linter

[jyaistMap]

THANK YOU, @xlt208