[docs][core] Documentation improvement in master/walkthrough.html (#12473)

doc/source/walkthrough.rst

@@ -290,6 +290,13 @@ Cancelling tasks
       obj_ref = blocking_operation.remote()
       ray.cancel(obj_ref)
 
+      from ray.exceptions import TaskCancelledError
+
+      try: 
+          ray.get(obj_ref)
+      except TaskCancelledError: 
+          print("Object reference was cancelled.")
+
   .. group-tab:: Java
 
     Task cancellation hasn't been implemented in Java yet.
@@ -354,7 +361,7 @@ If the current node's object store does not contain the object, the object is do
       from ray.exceptions import GetTimeoutError
 
       @ray.remote
-      def long_running_function()
+      def long_running_function():
           time.sleep(8)
 
       obj_ref = long_running_function.remote()
@@ -490,7 +497,7 @@ value.
 
     # Call the actor.
     obj_ref = counter.increment.remote()
-    ray.get(obj_ref) == 1
+    assert ray.get(obj_ref) == 1
 
   .. code-tab:: java