Fixed about_arrays.rb to pass all tests

Learned that:
- arrays belong to the Array class, and how to create a new array.
- there are more than one way to add new elements to array.
- array elemts can be accessed by using indexes.
- you can slice arrays by providing an index to start with 
  and a number of positions.
- ranges belong to Range class and you can create arrays from ranges.
- you can use '..' and '...' to create ranges. The latter won't include 
  the upper limit of the range.
- you can use ranges of indexes to slice an array.
- push, pop, shift and unshift array methods are used to manipulate the 
  array's elements.

[#65851746]