chore: remove repetitive word

Signed-off-by: acceptacross <csqcqs@gmail.com>
(cherry picked from commit 6f4bb8e)

Author: acceptacross

rayon-core/src/lib.rs

@@ -232,7 +232,7 @@ type DeadlockHandler = dyn Fn() + Send + Sync;
 type StartHandler = dyn Fn(usize) + Send + Sync;
 
 /// The type for a closure that gets invoked when a thread exits. The
-/// closure is passed the index of the thread on which is is invoked.
+/// closure is passed the index of the thread on which it is invoked.
 /// Note that this same closure may be invoked multiple times in parallel.
 type ExitHandler = dyn Fn(usize) + Send + Sync;