[docs] revert removal of newlines from html (#6386)

An unintended change was merged in #6062

Author: JBallin

docusaurus/docs/adding-custom-environment-variables.md

@@ -41,13 +41,19 @@ During the build, `process.env.REACT_APP_NOT_SECRET_CODE` will be replaced with
 
 When you load the app in the browser and inspect the `<input>`, you will see its value set to `abcdef`, and the bold text will show the environment provided when using `npm start`:
 
+<!-- prettier-ignore-start -->
+
 ```html
 <div>
   <small>You are running this application in <b>development</b> mode.</small>
-  <form><input type="hidden" value="abcdef" /></form>
+  <form>
+    <input type="hidden" value="abcdef" />
+  </form>
 </div>
 ```
 
+<!-- prettier-ignore-end -->
+
 The above form is looking for a variable called `REACT_APP_NOT_SECRET_CODE` from the environment. In order to consume this value, we need to have it defined in the environment. This can be done using two ways: either in your shell or in a `.env` file. Both of these ways are described in the next few sections.
 
 Having access to the `NODE_ENV` is also useful for performing actions conditionally: