docs: fix bad closed tag (#59575)
Found this from build error trace while testing the change in #59569
```
docs/02-app/01-building-your-application/08-testing/02-jest.mdx": UnexpectedMDXError: Error: Build failed with 1 error:
--
_mdx_bundler_entry_point-776983b1-6900-47c0-98cc-0c35882e9532.mdx:297:0: ERROR: [plugin: @mdx-js/esbuild] Unexpected closing tag `</PageOnly>`, expected corresponding closing tag for `<PagesOnly>
```
Closes NEXT-1863
