docs: update with-google-analytics example path (#70547)

## Description
[with-google-analytics
example](https://github.com/vercel/next.js/tree/canary/examples/with-google-analytics)
was updated for App Router at #66021.
So remove an old pages path.

### Improving Documentation

- [x] Run `pnpm prettier-fix` to fix formatting issues before opening
the PR.
- [x] Read the Docs Contribution Guide to ensure your contribution
follows the docs guidelines:
https://nextjs.org/docs/community/contribution-guide

---------

Co-authored-by: JJ Kasper <[email protected]>

docs/02-app/01-building-your-application/06-optimizing/08-analytics.mdx

@@ -206,7 +206,7 @@ useReportWebVitals((metric) => {
 > ```js
 > useReportWebVitals((metric) => {
 >   // Use `window.gtag` if you initialized Google Analytics as this example:
->   // https://github.com/vercel/next.js/blob/canary/examples/with-google-analytics/pages/_app.js
+>   // https://github.com/vercel/next.js/blob/canary/examples/with-google-analytics
 >   window.gtag('event', metric.name, {
 >     value: Math.round(
 >       metric.name === 'CLS' ? metric.value * 1000 : metric.value

docs/02-app/02-api-reference/04-functions/use-report-web-vitals.mdx

@@ -223,7 +223,7 @@ useReportWebVitals((metric) => {
 > ```js
 > useReportWebVitals(metric => {
 >   // Use `window.gtag` if you initialized Google Analytics as this example:
->   // https://github.com/vercel/next.js/blob/canary/examples/with-google-analytics/pages/_app.js
+>   // https://github.com/vercel/next.js/blob/canary/examples/with-google-analytics
 >   window.gtag('event', metric.name, {
 >     value: Math.round(metric.name === 'CLS' ? metric.value * 1000 : metric.value), // values must be integers
 >     event_label: metric.id, // id unique to current page load