Fix MIME type of JSON across all documentation pages.

Per RFC 4627, the MIME type of JSON is `application/json`.
However, the (commonly used) incorrect MIME type of
`text/json` is used in many places throughout the Grails
documentation.

Grails actually supports the correct MIME type, so there is no
reason to use the incorrect one in the documentation except in
exceptional circumstances, if any exist.

Additional information: http://stackoverflow.com/q/477816/612279