doc: fix doc for correct usage with plan & TestContext

Fixed section in the doc that describes a test that uses the ⁠plan
feature in the test-runner.
However, the test in this example fails.
The fix use  (Textcontext) and reduce the plan number
to 1 since we have 1 assertion.

PR-URL: #53615
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>

doc/api/test.md

@@ -3140,9 +3140,9 @@ behaves in the same fashion as the top level [`test()`][] function.
 test('top level test', async (t) => {
   await t.test(
     'This is a subtest',
-    { only: false, skip: false, concurrency: 1, todo: false, plan: 4 },
+    { only: false, skip: false, concurrency: 1, todo: false, plan: 1 },
     (t) => {
-      assert.ok('some relevant assertion here');
+      t.assert.ok('some relevant assertion here');
     },
   );
 });