You should mark this up with replaceable tags around the X, so that it is clear that this is the part the user has to change:
<filename>ttyU<replaceable>X</replaceable></filename>

the number of the device.

It's simpler to write:
Use <command>cu</command> to establish a connection through USB to UART.

I would also markup speed in <replaceable> tags here. Examples should be something a user could enter at the prompt and it would work, even though the speed is not the correct one. Having replaceable tags, you can reference it in the sentence following the example to change the speed to the one fitting the device.
For example, you can write: The following example connects to a Beaglebone Black using a baud-rate of 115,200: (example follows with that number in the example, surrounded by <replaceable> tags) Next paragraph: For other devices, replace the baud-rate with the one typically found on the device vendor documentation or website.

Needs a space between "Use" and <command>.

fixed