Fixes to about_arrays.rb

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.

[#65851746]
More fixes to about_arrays.rb

Learned that:
* 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.

[#65851746]
Yet some more fixes to about_arrays.rb

Learned that:
* 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]