getting_started/4: fix "data structured" typo.
getting_started/4: 4.2: reword head of 2nd paragraph.¹

1: I find this difficult to express in a clear manner.
getting_started/4: 4.2 - 4.2.3: reword sentences, tidy flow, correct IEx output, codify more syntax.
Merge pull request #143 from komiga/gs-4-tidy

Fix typos, codify syntax, reword & reflow sentences in Getting Started sections 4.2 - 4.2.3