update lesson 2

* fixed some minor issues
* short paragraph on commenting

I think we should introduce commenting as early as possible. @ashwinsrnth, should we have the 'publish'-section is this lesson?
I would also suggest that we change 'i' to 'idx', since 'i' is the imaginary unit in Matlab. 
Markdown has "smart" list numbering. So

~~~
1. Sample
1. Another Sample.
~~~

is equals to

~~~
1. Sample
2. Another Sample.
~~~