getting_started/1: codify "true" as it is elsewhere.
getting_started/2: fix "exemple" typo.
getting_started/2: fix "insted" typo.
getting_started/2: codify "true" as it is elsewhere.
getting_started/3: corrected output and use of "concat.ex".¹

1: Said to save code as "concat.ex", but then showed output and use of an
undeclared file "test_funs.ex"
Merge pull request #140 from komiga/master

Fix typos and normalize syntax highlighting in chapters 1, 2, and 3 of Getting Started