change content-type suggestion

avoid confusion like this: #81

Using `{'Content-Type': 'application/json'}` will result in "400 Bad Request: invalid header name" response

infrastructure/internal-api/code/index.html

@@ -41,7 +41,7 @@
                 <p class="font-semibold">Custom Header: </p><br>
                 <input
                     class="bg-white focus:outline-none focus:shadow-outline border border-gray-300 rounded-lg py-2 px-4 block w-full appearance-none leading-normal header"
-                    type="text" placeholder="{ 'Content-Type': 'application/json' }" id="headers"><br><br>
+                    type="text" placeholder="Content-Type: application/json" id="headers"><br><br>
 
                 <button
                     class="bg-transparent hover:bg-blue-500 text-blue-700 font-semibold hover:text-white py-2 px-4 border border-blue-500 hover:border-transparent rounded submit-button"
@@ -101,4 +101,4 @@
 
 </body>
 
-</html>
+</html>