Avoiding 'i' as a variable in MATLAB materials

* replaced ‘i’ with ‘idx’ in lesson 2
* replaced ‘i’ with ‘j’ in lesson 4

'i' is used as the imaginary unit in MATLAB. I think it's better style to avoid using it as a variable, too. 

@ashwinsrnth, in lesson 4, is it possible that 'i' was meant to be '1'?
The second argument to `sprintf` should now be `idx`