fix: React warning related to "true" used as boolean (via #5497)

This should fix this warning seen in the console when using `swagger-ui-react`:

```
Warning: Received the string `true` for the boolean attribute `readOnly`. Although this works, it will not work as expected if you pass the string "false". Did you mean readOnly={true}?
    in textarea (created by Curl)
    in div (created by Curl)
    in div (created by Curl)
    in Curl (created by LiveResponse)
    in div (created by LiveResponse)
    in LiveResponse (created by Responses)
```

I did not test this change because it's trivial and there is no behavioural change here, React treats non-empty string attributes as `true`.

src/core/components/curl.jsx

@@ -20,7 +20,7 @@ export default class Curl extends React.Component {
       <div>
         <h4>Curl</h4>
         <div className="copy-paste">
-          <textarea onFocus={this.handleFocus} readOnly="true" className="curl" style={{ whiteSpace: "normal" }} value={curl}></textarea>
+          <textarea onFocus={this.handleFocus} readOnly={true} className="curl" style={{ whiteSpace: "normal" }} value={curl}></textarea>
         </div>
       </div>
     )