Update preparing_the_build.md
Created by: adrianN
mdBook uses pulldown-cmark. pulldown-cmark implements CommonMark. CommonMark, unlike Github Markdown, needs a space after # for ATX style headers. See http://spec.commonmark.org/0.27/#atx-headings
Created by: adrianN
mdBook uses pulldown-cmark. pulldown-cmark implements CommonMark. CommonMark, unlike Github Markdown, needs a space after # for ATX style headers. See http://spec.commonmark.org/0.27/#atx-headings