Spring Improve example in Javadoc for HttpEntity

There is a minor typo in java doc. To be specific, provided code snippet in HttpEntity does not work because of lack of quotation marks.

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.TEXT_PLAIN);
HttpEntity<String> entity = new HttpEntity<String>(helloWorld, headers);
URI location = template.postForLocation("https://example.com", entity);

Affects: \<5.3.10>

Comment From: sbrannen

Hi @no-brand,

Thanks for creating your first pull request for the Spring Framework! 👍

It is assumed that helloWorld is a variable of type String defined elsewhere, but you're right that the example can be improved. So we'll improve it.