Missing install of @babel/core (webdriverio#3258)

## Proposed changes

The babel docs missed installing of the core

## Types of changes

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- [x] Bugfix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

## Checklist

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- [ ] I have added tests that prove my fix is effective or that my feature works
- [ ] I have added necessary documentation (if appropriate)

## Further comments

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docs/Babel.md

@@ -6,7 +6,7 @@ title: Babel Setup
 To write tests using next generation JavaScript features you can add [Babel](https://babeljs.io/) as compiler for your test files. For that first, install the necessary Babel dependencies:
 
 ```
-npm install --save-dev @babel/cli @babel/preset-env @babel/register
+npm install --save-dev @babel/core @babel/cli @babel/preset-env @babel/register
 ```
 
 Make sure your [`babel.config.js`](https://babeljs.io/docs/en/config-files) is configured properly. The simplest setup you can use is: