Make `c` example valid

- there was a `var` declaration
- `printf` didn't  have format
- `printf` was going to print 10 instead of 9
- an empty comment didn't make the error obvious
- combining `var` and `int` made some students believe the code was in js and later expected js to have a `prinf` built-in function
- added em and strong to emphasize the explanation
- on the next slide there was a missing comma