fix(readme): tidy up markup of nested code blocks

Using solution from https://stackoverflow.com/a/42320033/325365, credits go to @B4nan

Author: honzajavorek

README.md

@@ -43,15 +43,15 @@ Instead, try to be more descriptive, for example:
 - For inline `code` examples, use **back-ticks** (\` \`).
 - For multi-line code examples, use code fences and specify the language. Preferably, specify the title as well. Within platform documentation, always use [Code tabs](README.md#code-tabs) and declare the language.
 
-  ```markdown
-  \`\`\`js showLineNumbers title='foo.js'
+  ``````markdown
+  ```js showLineNumbers title='foo.js'
 
   const docsAreCool = require('coolDocs'); <br/>
   ...<br/>
   return docsAreCool;<br/>
 
-  \`\`\`
   ```
+  ``````
 
 Check out [Markdown features](https://docusaurus.io/docs/markdown-features) in the Docusaurus docs for more information.