Command formatting issue

[mafirenze]

Thank you for making all these webinars available they are incredibly helpful!

A small issue I found:
In the tutorial: bart-webinars/webinar4/demo /demo.ipynb

All the commands that use rss give an error in my version and in the binder interactive version.
For example the line: "im_rss = bart.bart(1, 'rss $(bart bitmask 3)', im_coil)",

give an error like Usage: rss bitmask
ERROR: cmdline: too few or too many arguments
Command exited with error code -6.

To fix this I found that separating the command in two lines, this way, works :
num = !bart bitmask 3
im_rss = bart.bart(1, 'rss {}'.format(num[0]), im_coil)

I'm not sure why this is but may be something related to newer python