Update semaphore.rs

Co-authored-by: Alice Ryhl <[email protected]>

tokio/src/sync/semaphore.rs

@@ -340,7 +340,7 @@ use std::sync::Arc;
 /// completed before continuing.
 ///
 /// The task awaiting the countdown should call [`Semaphore::acquire_many`] to
-/// acquire the a number of permits equal to the number of tasks which hold the
+/// acquire a number of permits equal to the number of tasks which hold the
 /// latch open. The tasks whose completion is awaited by the latch should call
 /// [`Semaphore::add_permits`] with a single permit to increment the countdown
 /// latch as they complete. For example: