fix(docs): code blocks in ch02-02-functions of the book

[benediktwerner]

Markdown doesn't close code-blocks when the closing ``` isn't on a separate line.

Rendering the change shows the difference and it also happens in the rendered book.

[codecov]

Codecov Report

Merging #194 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #194   +/-   ##
=======================================
  Coverage   82.94%   82.94%           
=======================================
  Files         163      163           
  Lines       11509    11509           
=======================================
  Hits         9546     9546           
  Misses       1963     1963           

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[baszalmstra]

Thanks very much! 👍