docs(nixtlaverse): fix preposition typo on home page

[loama]

What

Fixes a small grammar typo on the Nixtlaverse landing page (index.mdx): "composed by" → "composed of".

Why

Found during the 2026-07-19 daily site QA review. "The Nixtlaverse is composed by our open-source libraries" reads as a translation error; "composed of" is the correct preposition.

How

• One-line text change in index.mdx.

Testing

• Visual diff only, no logic change.
• Manual: confirm the sentence reads correctly on the deployed preview.

Checklist

• Title follows conventional commits format
• PR is focused on a single concern
• Self-reviewed the diff before requesting review
• No secrets, credentials, or PII in the diff

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Project	Status	Preview	Updated (UTC)
nixtlaverse_nixtla_io	🟢 Ready	View Preview	Jul 22, 2026, 3:13 PM

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[loama]

The failing "Mintlify Validation - link-rot" check reports 25 broken links, all in unrelated files (mlforecast/, neuralforecast/, statsforecast/, utilsforecast/ docs — missing images and cross-references). None are in index.mdx, the only file this PR touches (a one-line preposition fix). This is pre-existing repo-wide link rot, not something this PR introduces; flagging separately rather than expanding this PR's scope to fix it.