docs: use "length" over "len" in tutorial3 python

[alestiago]

Description

Edit the occurrence of "len" to "length" since the ble_command_get_version.py that is being referenced uses "length".

See permalink:

OpenGoPro/demos/python/tutorial/tutorial_modules/tutorial_3_parse_ble_tlv_responses/ble_command_get_version.py

Line 40 in f36409a

while index <= length:

[alestiago]

@tcamise-gpsw , first of all thank you for maintaining this repository.

This is a simple Pull Request, when I was reading through the tutorials I noticed this mismatch between the variable names in the docs and those in the tutorial (demo).

The CI seems to be failing on the "Pre Merge Checks", if there is something I need to do on my end let me know. Feel free to close or amend yourself.

Once again, thank you for maintaining this and I hope to see GoPro get more involved in OSS.