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.

Abre BUILDING.md e inspecciona la línea anterior al primer encabezado de nivel 1 y la línea posterior al párrafo final. Corrige los problemas de sintaxis de Markdown y cambia "it's" por "its" en el primer párrafo de Angular CLI. La tarea estará terminada cuando se hayan eliminado la regla horizontal no intencionada y el encabezado de párrafo, y se haya corregido la redacción.