chore: remove repetitive word

Signed-off-by: acceptacross <[email protected]>

rayon-core/src/lib.rs

@@ -228,7 +228,7 @@ type PanicHandler = dyn Fn(Box<dyn Any + Send>) + 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;