Flashing esp32-s3-usb-otg - add information to hold boot button

[georgik]

If CircuitPython is flashed on the board, it's necessary to restart the board and hold boot button.

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Error from console

port: /dev/cu.usbserial-12110
Connecting to port...
Unable to connect, retrying with extra delay...
Unable to connect, retrying with default delay...
Unable to connect, retrying with extra delay...
Unable to connect, retrying with default delay...
Unable to connect, retrying with extra delay...
Unable to connect, retrying with default delay...
Unable to connect, retrying with extra delay...
thread 'tokio-runtime-worker' panicked at src/flasher.rs:117:81:
called `Result::unwrap()` on an `Err` value: Connection(ConnectionFailed)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace