scripts docs:api remove not exist path

[ro7584]

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

rimraf command remove not exist path.
https://github.com/mui-org/material-ui/blob/master/package.json#L9

Expected Behavior 🤔

According to faca5c4 change docs/pages/api to docs/pages/api-docs
Maybe it should update rimraf command as well.

-"docs:api": "rimraf ./docs/pages/api && cross-env BABEL_ENV=test __NEXT_EXPORT_TRAILING_SLASH=true babel-node ./docs/scripts/buildApi.js ./packages/material-ui/src ./docs/pages/api-docs && cross-env BABEL_ENV=test __NEXT_EXPORT_TRAILING_SLASH=true babel-node ./docs/scripts/buildApi.js ./packages/material-ui-lab/src ./docs/pages/api-docs",
+"docs:api": "rimraf ./docs/pages/api-docs && cross-env BABEL_ENV=test __NEXT_EXPORT_TRAILING_SLASH=true babel-node ./docs/scripts/buildApi.js ./packages/material-ui/src ./docs/pages/api-docs && cross-env BABEL_ENV=test __NEXT_EXPORT_TRAILING_SLASH=true babel-node ./docs/scripts/buildApi.js ./packages/material-ui-lab/src ./docs/pages/api-docs",

[eps1lon]

Feel free to open a PR 👍