* Fix installation.md typo
Checking the docusaurus version should be done with
```
npx docusaurus --version
```
not
```
npm docusaurus --version
```
otherwise it prints the version of `npm` instead of `docusaurus`.
* docs: Fixed installation.md for checking version.
Specifying `npm` for the version will yield the version of NPM
instead of docusaurus itself. Use `npx` for the docusaurus version.