`ZolotoCameraBoard.save` does not return a value

[PeterJCLaw]

In programming/sr/vision/index.md we have the following example which is wrong:

from sr.robot3 import *
R = Robot()

# `R.usbkey` is the path to your USB drive
marker_ids = R.camera.save(R.usbkey / "initial-view.png")

The save method does not return anything.

Found as a result of running mypy on the docs' code examples, having extracted them using https://gist.github.com/PeterJCLaw/318d392c46daff36577df5e24866d5b0 while working on srobo/competition-simulator#350.