Typo fix

Author: petertodd

src/libcore/mem/manually_drop.rs

@@ -121,7 +121,7 @@ impl<T: ?Sized> ManuallyDrop<T> {
     /// This function runs the destructor of the contained value and thus the wrapped value
     /// now represents uninitialized data. It is up to the user of this method to ensure the
     /// uninitialized data is not actually used.
-    /// In particular, this function can only be called called at most once
+    /// In particular, this function can only be called at most once
     /// for a given instance of `ManuallyDrop<T>`.
     ///
     /// [`ManuallyDrop::into_inner`]: #method.into_inner