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.

BUILDING.md を開き、最初のレベル 1 見出しの前の行と、最後の段落の後の行を確認します。Markdown の構文上の問題を修正し、最初の Angular CLI の段落で "it's" を "its" に変更します。意図しない水平線と段落見出しが削除され、文言が修正されれば完了です。