Fix name of "ai-assisted" label in llm policy

[steffahn]

The actual label we created is called llm-assisted now.

It's a minimal change, but IMO this point was sufficiently confusing in practice that it's worth fixing sooner rather than later.
This is of course a minor change, following the policy's conditions for modification

Minor changes, such as typo fixes, only require a normal PR approval.

I've also opened a PR to fix the llm guidance in the dev-guide: rust-lang/rustc-dev-guide#2962

Rendered

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