Fix async project goal link

[zjncs]

Problem

src/intro.md links to the async Rust project goal at https://github.com/rust-lang/rust-project-goals/issues/105. That repository has been renamed to rust-lang/goals, so the old URL now returns 404 (GitHub does not redirect renamed repos outside the same owner's redirect window once the name is reused — verified: old URL 404, new URL 200).

Change

Update the one link in src/intro.md to point to https://github.com/rust-lang/goals/issues/105.

Verification

• Base SHA: 43891ce (tip of master at the time of the change; still the tip).
• grep -rn "rust-project-goals" — no other references remain in the book.
• Old URL returns HTTP 404; new URL returns HTTP 200.
• mdbook build succeeds (book builds with the same warnings as before the change).
• The examples test suite passes: 13 tests passed, only pre-existing warnings.

Risk

Minimal: a one-line documentation link fix.

AI assistance disclosure

This patch was prepared by an autonomous AI coding agent at the direction of the account owner.

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