I figured it out, after beating my head further against the commandline, which does not function like the instructions.  I did the following command: "boot system unit 3 image-2" (where the unit number equals the switch in the stack) This doesn't even come close to matching the documentation, but it works.  I hope this is helpful to someone else.
I'm upgrading the OS in several switches in one of our buildings.  One pair is a set of stacked 3548P switches running version 2.0.0.47.  Oddly (in case this has any reference) the switches have stack IDs of 1 and 3 (there is no ID 2). I've backed up the configs, and already uploaded the new image (2.0.0.53) and boot code.  However, when using Dell's following commands as described in their guide: boot system image-2 switch 1 I get the following response: "Wrong number of parameters or invalid range, size, or characters entered" I then did just "boot system-image 2".  This actually allowed the first switch in the stack to change its boot image.  However, I cannot do a "boot system image-2 switch 3" I get the same error.  I have also tried "boot system image-2 all".  Same error. I'm not going to reboot the switches until I can ensure I'm not going to break something, but I'm not sure what I can do to resolve the issue.  I'm following the documentation to the letter as far as I can tell.  Does anyone have any ideas?
Console error when upgrading stacked PowerConnect 3548P switches