Update RUM Recommendations URL

[rtrieu]

What does this PR do? What is the motivation?

URL should be ~/recommendations instead of ~/rum_recommendations

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

✅ Documentation Team Review

The documentation team has approved this pull request. Thank you for your contribution!

[github-actions]

Preview links (active after the build_preview check completes)

Renamed files

• https://docs-staging.datadoghq.com/rtrieu/update-rum-recommendations-url/real_user_monitoring/browser/optimizing_performance/recommendations