docs: fix assertion example with correct condition

From the context of the text, the `assert` condition should include a field property above.

Author: melosa11

doc/learning/tutorial.html

@@ -493,7 +493,7 @@ <h2 id="errors">Errors</h2>
         <li>To assert a condition before an expression: <code>assert "foo";</code></li>
         <li>A custom failure message: <code>assert "foo" : "message";</code></li>
         <li>Assert fields have a property: <code>assert self.f == 10,</code></li>
-        <li>With custom failure message: <code>assert "foo" : "message",</code></li>
+        <li>With custom failure message: <code>assert self.f == 10 : "message",</code></li>
       </ul>
       <p>
         Try modifying the code below to trigger the assertion failures, and observe the error