[valhalla-docs] RFR: site/_index.md: Fix JEP 401 link

[Christopher Sahnwaldt]

typo: link should point to 401, not 390

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Commit messages:
 - site/_index.md: remove line break added by GitHub editor
 - try to remove line break added by GitHub editor
 - site/_index.md: Fix JEP 401 link

Changes: https://git.openjdk.org/valhalla-docs/pull/15/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla-docs&pr=15&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla-docs/pull/15.diff
  Fetch: git fetch https://git.openjdk.org/valhalla-docs.git pull/15/head:pull/15

PR: https://git.openjdk.org/valhalla-docs/pull/15