Fix typo in scheduler docstring

Author: iyad-f

src/async_utils/scheduler.py

@@ -49,7 +49,7 @@ def __lt__(self, other: _Task[T]) -> bool:
 class Scheduler[T]:
     """A scheduler.
 
-    The scheduler is implemented as an async context manager that it an
+    The scheduler is implemented as an async context manager that is an
     async iterator.
 
     Payloads can be scheduled to the context manager, and will be yielded