Fix integrations example

[carestad]

The example used (call to new Sentry.BrowserTracing()) gave errors in the browser. The correct code seems to be new Sentry.Integrations.BrowserTracing().

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🔍 Inspect: https://vercel.com/sentry/sentry-docs/72txthSkP2os9AC3xs4XxtuorT25
✅ Preview: https://sentry-docs-git-fork-carestad-patch-2.sentry.dev

[lobsterkatie]

Hi, @carestad! Thanks for your contribution!

We changed how BrowserTracing is accessed, so the docs are right, or at least, describe what should be right. Unfortunately, the CDN bundle was missed in that PR. I've pushed a new PR to fix this, linked above.