Improve data-structures/heap/README.md (#94)

* Update data-structure/heap/README.md

The overuse of parenthesis in the previous description made the
explanation of a heap more diffcult to read. Following technical
writing standards, the topic of the sentence should be at the
forefront of a sentence, to give the reader an idea of what is
going to be explained in said sentence.

The mention of min heap and max heap were previously in parenthesis, in
the middle of the sentence. This change informs the reader that there
are two types of heaps, with a preface indicating which one is being
explained.

* Add min heap picture and move max heap picture

Add a picture of a min heap below the explanation of a min heap.
Move the picture of a max heap below the explanation of a max heap.

* Italicize heap terminology

Italicize the terms 'max heap' and 'min heap' to make it clear to readers that new terminology is being introduced