BUILDING.md: minor typo and some Markdown syntax errors

Markdown interprets a line composed of three or more dashes as a horizontal rule if the prior line is blank. A line of one or more dashes preceded by a non-blank line makes that prior line a level-1 header.

In BUILDING.md, there's a line of three dashes immediately preceding the first level-1 header (Angular CLI), which is interpreted as a horizontal rule. I'm not convinced that was intended. More serious is the fact that the final paragraph is immediately followed by a line of three dashes, making the entire paragraph a level-1 header — a definite mistake.

Additionally, the first paragraph of Angular CLI uses "it's" where "its" should be used.

I can create a PR if that would help.

Ouvrez BUILDING.md et examinez la ligne précédant le premier en-tête de niveau 1 ainsi que la ligne suivant le dernier paragraphe. Corrigez les problèmes de syntaxe Markdown et remplacez "it's" par "its" dans le premier paragraphe consacré à Angular CLI. La tâche est terminée lorsque la règle horizontale et l'en-tête de paragraphe involontaires ont été supprimés et que la formulation a été corrigée.