Small fixes to the bhyve handbook section about nmdm devices and default cu baud rate

[bcr]

Summary

This is a fix to section 21.7.5. Virtual Machine Consoles:
The paragraph talks about switching to /dev/nmdm0A for the null modem cable. But then, when describing the two ends of it, /dev/nmdm1A and /dev/nmdm1B are used. I think this leads to confusion and so this patch changes this to always use /dev/nmdm0[AB].
Or is there a reason to use /dev/nmdm1[AB] all of a sudden that I don't see?

The other change trims down the speed option of the cu line. According to cu(1):
-s speed | -speed

	     Set the speed of the connection.  The default is 9600.

So it is not required to specify the baud rate of 9600 here, as it is the default.

Test Plan

1. Apply the patch
2. build the handbook
3. Look at the output for that section

[allanjude]

Originally there were 2 separate examples, so nmdm0 and nmdm1, but then it was reduced and left slightly confusing like that.

thanks for fixing.