chore(docs): update getting-started.md (#4287)

This PR updates the "locally" world to "globally", because the subsequent command installation has this global `-g` flag:

```bash
npm install netlify-cli -g
```

At least, that's how I see it. It was a bit confusing at me first when the caption is "locally" but the code snippet after has this `-g` flag. If this PR doesn't make sense, please feel free to close this. Thanks!

docs/getting-started.md

@@ -10,7 +10,7 @@ This guide will walk you through installing and creating your first site with th
 
 Lets get rolling with the Netlify CLI.
 
-1. **Install the CLI locally**
+1. **Install the CLI globally**
 
    ```bash
    npm install netlify-cli -g