Changed type of {id} in submission form to number

[hbina]

Fixes the issue where the user enters a string and it will returns error 400 because it can't validate object Greeting of a string id.
Apologies for the duplicate Issues, wonder if I can remove the old one?

[pivotal-issuemaster]

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[gregturn]

This PR contains two changes.

• It changes the HTML input type from text to number. This is valid in the sense that yes, ids are numbers. And on certain devices, perhaps a smartphone, it would show a numeric input. But this example actually doesn't look since this form really supports editing a database id. A better cleanup would be to NOT show the id, but instead to show something price instead.
• Alterations to the POJO and how it is passed back and forth to the form. I don't really favor renaming something to GreetingClass. It is ALREADY a class, so why embed that in its name. Also, the template is greeting.html in the folder src/main/resources/templates, hence we know it's a template. So again, no need to embed such semantic information that is already in the context.

The former change I would be open toward collaborating on a clean pull request to apply.

For the latter, I don't think we are going to go with it. If you feel the wording in the guide is lacking, by all means, submit a pull request with proposed changes to clarify how Java beans map onto forms with Thymeleaf.