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.

打开 BUILDING.md，检查第一个一级标题之前的行以及最后一个段落之后的行。修正 Markdown 语法问题，并将第一个 Angular CLI 段落中的 "it's" 改为 "its"。完成的标准是删除意外的水平线和段落标题，并修正文案。