README.md: Convert 'boot camp' -> 'workshop'

README.md: Convert 'boot camp' -> 'workshop'

Following Amy Brown's [1]:

> We borrowed the "bootcamp" terminology from Hacker Within and also
> various fitness training camps, but as we branch out into parts of
> the world with a more violent recent history I'm less comfortable
> with the term.

The consensus on that issue was to change back to our old 'workshop'
terminology.  The swcarpentry/bc repository hasn't made the shift yet,
but there's no reason I can't embrace the future here ;).

[1]: https://github.com/swcarpentry/bc/issues/240