chore(serverless): Fix incorrect path in Readme

[Lms24]

This patch fixes an incorrect path for requiring the auto-init script in the @sentry/serverless Readme for users who want to set up the layer via the ARN and environment variables. Furthermore, this patch slightly enhances the step to find the correct ARN by linking to our (still kinda broken) ARN dropdown in the docs.

resolves #6204

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.51 KB (+0.07% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	60.36 KB (+0.05% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.17 KB (+0.08% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	53.71 KB (+0.03% 🔺)
@sentry/browser - Webpack (gzipped + minified)	19.9 KB (+0.04% 🔺)
@sentry/browser - Webpack (minified)	65.12 KB (+0.04% 🔺)
@sentry/react - Webpack (gzipped + minified)	19.93 KB (+0.07% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	45.89 KB (+0.03% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.33 KB (+0.04% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.74 KB (+0.06% 🔺)