example: fix typo `_app_.tsx` → `_app.tsx` (#71686)
## Wrong
https://github.com/vercel/next.js/blob/canary/examples/with-segment-analytics-pages-router/pages/_app_.tsx
## Correct
https://github.com/vercel/next.js/blob/canary/examples/with-segment-analytics-pages-router/pages/_app.tsx
### Adding or Updating Examples
- [x] The "examples guidelines" are followed from our contributing doc
https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
- [x] Make sure the linting passes by running `pnpm build && pnpm lint`.
See
https://github.com/vercel/next.js/blob/canary/contributing/repository/linting.md
Co-authored-by: JJ Kasper <jj@jjsweb.site>
