doc: fix wrong function name in example of context.plan()

t.subtest -> t.test

Refs: nodejs#52860
PR-URL: nodejs#53140
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>

doc/api/test.md

@@ -3084,7 +3084,7 @@ expected count, the test will fail.
 test('top level test', (t) => {
   t.plan(2);
   t.assert.ok('some relevant assertion here');
-  t.subtest('subtest', () => {});
+  t.test('subtest', () => {});
 });
 ```