修正unreachable行为描述译文中指代不清的表达

[Sunshine40]

The phrase “unchecked” is a Rust idiom; this is the “unchecked” version of unreachable, which inserts a run-time check that safely aborts the program should this code ever be reached — or, to be more precise, it triggers a Rust panic.

原文的which从句描述的是 unreachable 的行为，完全保留语序直译为中文后指代产生了歧义。