Docs: fix a command bug

[ChungZH]

Summary
Fix a command bug in Chinese doc.

Before: yarn build:docs & npm run build:docs

After: yarn docs:build & npm run docs:build

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[billyyyyy3320]

It totally depends on what the property name you set to your NPM scripts

[billyyyyy3320]

Oh, I get it. The example commands isn't sync between zh and en docs.

[ChungZH]

Haha, that's it 😄
Actually, this is not only isn't sync, this command is wrong.
Thx!

[meteorlxy]

Sorry didn't notice this PR. Has already merged #2284