Upload "upgrade" docstring correction

[JPHutchins]

The docstring here:

smpclient/smpclient/__init__.py

Lines 217 to 222 in 334f5bc

	upgrade: `True` to mark the image as confirmed. This is unsafe and
	can cause a boot-loop that could brick the device. This setting
	should be left at the default `False` and the image should be
	confirmed from within the upgraded application. Zephyr provides
	[boot_write_img_confirmed()](https://docs.zephyrproject.org/apidoc/latest/group__mcuboot__api.html#ga95ccc9e1c7460fec16b9ce9ac8ad7a72)
	for this purpose.

Does not match what this option actually does: https://github.com/JPHutchins/smp/blob/c9bbef07641a62792e3f8ec31f64b11d9cca8107/smp/image_management.py#L195-L202

This upload command should not confirm, that's a separate command.