* Correct npm run tsc to npm run typecheck
According to this page, you should run `npm run tsc` to run `tsc` and do a type check. However, in the package.json file for Docusaurus version 2.0.0-beta.17 the command is actually `npm run typecheck`, which runs `tsc`.
This update only replaces `tsc` with `typecheck` so the npm script will run correctly.
* Recommend npx tsc for type-checking
Based on feedback for the original change to replace `npm run tsc` with `npm run typecheck`, a better solution that was suggested was to use npx to run tsc instead of an npm script. `npx tsc` should work regardless of the template/presets you installed when you installed Docusaurus.
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