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.

Abra BUILDING.md e inspecione a linha antes do primeiro cabeçalho de nível 1 e a linha depois do parágrafo final. Corrija os problemas de sintaxe do Markdown e altere "it's" para "its" no primeiro parágrafo do Angular CLI. A tarefa estará concluída quando a regra horizontal não intencional e o cabeçalho de parágrafo forem removidos e a redação for corrigida.