docs: fix README safety wording wrap

[abrichr]

Summary

Fix one Markdown line-wrap artifact in the newly rendered flagship README:
Safety-\ncritical rendered as “Safety- critical” on GitHub.

The replacement sentence keeps the same product meaning and renders as:

Local safety-critical verification is not paywalled.

Validation

• diff is one sentence only
• git diff --check
• rendered through GitHub's Markdown API with no Safety- critical text
• exact post-merge main checks for docs: rebuild flagship README around first value #1047 were already green before this fix:
  CodeQL 30137862118, secret scan 30137862111, release/no-op
  30137862152, docs notification 30137862109, and Pages 30137861681