[miri][typo] Fix a typo in a vector_block comment.

Author: ttsugriy

src/tools/miri/src/concurrency/vector_clock.rs

@@ -9,7 +9,7 @@ use std::{
 
 /// A vector clock index, this is associated with a thread id
 /// but in some cases one vector index may be shared with
-/// multiple thread ids if it safe to do so.
+/// multiple thread ids if it's safe to do so.
 #[derive(Clone, Copy, Debug, PartialOrd, Ord, PartialEq, Eq, Hash)]
 pub struct VectorIdx(u32);