docs: fix incorrect sample code

It is not valid that "application/json" and "GET".

Author: kenjis

user_guide_src/source/general/ajax.rst

@@ -18,7 +18,7 @@ Fetch API
 .. code-block:: javascript
 
     fetch(url, {
-        method: "get",
+        method: "POST",
         headers: {
           "Content-Type": "application/json",
           "X-Requested-With": "XMLHttpRequest"