Correct factual inaccuracies about Sentry in '10 Best Sentry Alternatives' (#140)

[simlarsen]

Addresses factual inaccuracies about Sentry flagged in #140 by @sergical (who works at Sentry). The original validation only fact-checked Datadog claims; the Sentry claims were never verified. All four were checked against official Sentry sources and corrected, while keeping the post a fair comparison that still discusses legitimate reasons to consider alternatives.

1. "Just an error tracker, not observability." Sentry provides tracing, profiling, session replay, cron monitoring, logs, and metrics with native correlation. Removed the "stitching tools" framing; reframed as a breadth-vs-single-platform trade-off. Sources: https://docs.sentry.io/product/tracing/ , https://docs.sentry.io/product/profiling/ , https://docs.sentry.io/product/session-replay/ , https://docs.sentry.io/product/crons/ , https://docs.sentry.io/product/explore/logs/
2. "SDKs are proprietary." False - sentry-javascript and sentry-python are MIT-licensed, and Sentry supports OpenTelemetry ingestion. Removed the claim; now states the SDKs are open source. Sources: https://github.com/getsentry/sentry-javascript/blob/develop/LICENSE , https://github.com/getsentry/sentry-python/blob/master/LICENSE , https://docs.sentry.io/platforms/javascript/guides/node/opentelemetry/
3. Pricing safeguards omitted. Added that Sentry offers spike protection, spend allocation, and an on-demand spending cap. Sources: https://docs.sentry.io/pricing/quotas/spike-protection/ , https://docs.sentry.io/pricing/quotas/spend-allocation/ , https://docs.sentry.io/pricing/quotas/
4. Self-hosting framing. Kept as a real consideration but noted most comparable self-hosted platforms have similar infrastructure needs.

Title, Tags, Description, and the coverage of the alternatives are unchanged. npm run validate passes.

Resolves #140