README.md direct URL example

[chaseadam]

Attempting to use the "direct URL" example and ran into several issues:

• It uses a non-raw URL example which presumable is unusable, but also results in very large payload, triggering a memory error
• Tip of master requires user and repo as arguments, but they are missing
• Extra / separator at the end

Seems /blob urls may have worked in the past, but no longer work for raw files.

Non-working:

GITHUB_URL = "https://github.com/RangerDigital/senko/blob/master/examples/"
OTA = senko.Senko(url=GITHUB_URL, files=["boot.py", "main.py"])

Working:

GITHUB_URL = "https://raw.githubusercontent.com/RangerDigital/senko/master/examples"
OTA = senko.Senko(None, None, url=GITHUB_URL, files=["boot.py", "main.py"])