doc: fix wrong function name in example of `context.plan()`
t.subtest -> t.test
Refs: https://github.com/nodejs/node/pull/52860
(1) With t.subtest
✖ top level test (2.496498ms)
TypeError: t.subtest is not a function
at TestContext.<anonymous> (/home/deokjinkim/oss/node-playground/tr2.js:6:5)
at Test.runInAsyncScope (node:async_hooks:206:9)
at Test.run (node:internal/test_runner/test:841:25)
at Test.processPendingSubtests (node:internal/test_runner/test:550:18)
at node:internal/test_runner/harness:247:12
at node:internal/process/task_queues:140:7
at AsyncResource.runInAsyncScope (node:async_hooks:206:9)
at AsyncResource.runMicrotask (node:internal/process/task_queues:137:8)
(2) With t.test
▶ top level test
✔ subtest (1.334498ms)
▶ top level test (3.065425ms)
ℹ tests 2
ℹ suites 0
ℹ pass 2
ℹ fail 0
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 12.305652