update the LTI postMessage doc for 'lti.fetchWindowSize' message
update the LTI postMessage doc for 'lti.fetchWindowSize' message

why:
  The documentation for the window.postMessage in LTI tools for
  the 'lti.fetchWindowSize' message is outdated

refs INTEROP-7199
flag=none

--------------------------------------------------
Test plan
--------------------------------------------------
* launch the LTI 1.3 Test Tool from any placement

* open the Browser Developer Tools (F12)

* in the "Send a window.postMessage" section, use the JSON:
   `{ "subject": "lti.fetchWindowSize" }`

* after sending, check the Console tab of the Browser Developer
  Tools

* it should have the Object below with the message sent to Canvas
   ```
   {
     "messageSent": {
       "message": {
         "subject": "lti.fetchWindowSize"
       },
       "targetOrigin": "*"
     }
   }
   ```

* it should have an Object like the one below with the message
  received from Canvas
   ```
   {
     "messageReceived": {
       "data": {
         "subject": "lti.fetchWindowSize.response",
         "height": 531,
         "width": 657,
         "offset": {
           "top": 79.20000457763672,
           "left": 24
         },
         "footer": 0,
         "scrollY": 79.33333587646484
       },
       "origin": "http://canvas.docker"
     }
   }
   ```

Change-Id: I96183787c3ebc01c4757334a0dd38e0641c6c0e9
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/290234
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Tucker Mcknight <4b51b98c1d84aa97854d216bd061ce5d3eda8a28@instructure.com>
Reviewed-by: Alexandre Trindade <f185c528be8ad1468cdf5c6cccdc72d6de8c2461@instructure.com>
Product-Review: Alexis Nast <0a669b9d4474d12a832cfeaf827a9e7c64df0a2a@instructure.com>
QA-Review: Alexandre Trindade <f185c528be8ad1468cdf5c6cccdc72d6de8c2461@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms