Updated wrong link to example of gtag init in measuring-performance.md (

#32974)

Updated wrong link to example of gtag init



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ x ] Make sure the linting passes by running `yarn lint`

docs/advanced-features/measuring-performance.md

@@ -164,7 +164,7 @@ export function reportWebVitals(metric) {
 > ```js
 > export function reportWebVitals({ id, name, label, value }) {
 >   // Use `window.gtag` if you initialized Google Analytics as this example:
->   // https://github.com/vercel/next.js/blob/canary/examples/with-google-analytics/pages/_document.js
+>   // https://github.com/vercel/next.js/blob/canary/examples/with-google-analytics/pages/_app.js
 >   window.gtag('event', name, {
 >     event_category:
 >       label === 'web-vital' ? 'Web Vitals' : 'Next.js custom metric',