Switched media type to application/json

docs/src/main/asciidoc/spring-cloud-netflix.adoc

@@ -1652,7 +1652,7 @@ class MyFallbackProvider implements ZuulFallbackProvider {
             @Override
             public HttpHeaders getHeaders() {
                 HttpHeaders headers = new HttpHeaders();
-                headers.setContentType(MediaType.TEXT_HTML);
+                headers.setContentType(MediaType.APPLICATION_JSON);
                 return headers;
             }
         };