fix: separated code blocks for the React plugin example

[plsalvado]

Short Description

Separated code block example under the React Plugin section into 2 code blocks, one for plugin registration and another for plugin invocation.

Relevant Links

https://deploy-preview-349--webiny-docs.netlify.app/docs/key-topics/plugins

Checklist

• I added page metadata (description, keywords)
• I've added "Can I Use This?" section (if needed, e.g. if documenting a new feature)
• I added What You'll Learn at the top of the page and every item in the list starts with a lower case letter
• I used title case for titles and subtitles (in the main menu and in the page content)
• I checked for typos and grammar mistakes
• I added alt / title attributes for inserted images (if any)
• When linking code from GitHub, I did it via a specific tag (and not next / v5 branch)

Screenshots (if relevant):