docs(v2): fix typo in command on installation page (#2886)

* 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.

website/docs/installation.md

@@ -123,7 +123,7 @@ npm install
 To check that that the update occurred successfully, run:
 
 ```bash npm2yarn
-npm docusaurus --version
+npx docusaurus --version
 ```
 
 You should see the correct version as output.

website/versioned_docs/version-2.0.0-alpha.56/installation.md

@@ -123,7 +123,7 @@ npm install
 To check that that the update occurred successfully, run:
 
 ```bash npm2yarn
-npm docusaurus --version
+npx docusaurus --version
 ```
 
 You should see the correct version as output.