Fixes best practices link in menu and avoid 3 warnings

The 'Best Practice' chapter has its menu link broken (in book.asciidoc) after build script is executed. The other changes are to avoid 3 warnings that appear during build:

asciidoc: WARNING: 05_proper.asciidoc: line 413: include file not found: /home/xxx/testing-erlang-book/proper/test_json_vs_python\.erl
asciidoc: WARNING: 11_best_practices.asciidoc: line 1: section title out of sequence: expected level 1, got level 2
asciidoc: WARNING: outline.asciidoc: line 4: section title out of sequence: expected level 1, got level 2