Well, when we use the word "diction" we actually refer to something as simple as the choice of words that an author used and the kind of effect that those words create. Therefore, if you are after examples of diction you can actually choose any part of the story you want as long as you comment on why the author chose those particular words and what he or she is trying to do to the reader with them.
One of my favourite parts of this story is the description of the T-Rex when he finally appears. Bradbury has carefully chosen his words to present us with a terrifying image of the might and majesty of this animal which so clearly overwhelms Eckels:
It came on great oiled, resilient, striding legs. It towered thirty feet above half of the trees, a great evil god, folding its delicate watchmaker's claws close to its oily reptilian chest. Each lower leg was a piston, a thousand pounds of white bone, sunk in thick ropes of muscle, sheathed over in a gleam of pebbled skin like the mail of a terrible warrior. Each thigh was a ton of meat, ivory, and steel mesh.
This description is of course just the beginning, but note how the word choice deliberately presents the T-Rex as a creature that cannot be killed. Its legs are described as "resilient," showing its determination and strength. It's description as a "great evil god" almost suggests that there is something supernatural about its appearance. The simile comparing the skin to "mail" adds to the impression of impermeability. The sheer weight of the thing and its strength causes us to question the impact that bullets can have.