Update controller.rst

Co-authored-by: Oskar Stark <oskarstark@googlemail.com>

Author: dbu

controller.rst

@@ -504,7 +504,7 @@ payload formats::
 
 .. tip::
 
-    If you write a json API, make sure to declare your route to use the json
+    If you write a JSON API, make sure to declare your route to use the JSON
     format. This will make the error handling output a json response in case of
     validation errors, rather than an HTML page.