Improve formatting of 'ManuallyDrop'

Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>

Author: Aaron1011

src/libcore/mem/manually_drop.rs

@@ -119,7 +119,7 @@ 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, and that this function is called at most once
-    /// for a given instance of ManuallyDrop
+    /// for a given instance of `ManuallyDrop<T>`.
     ///
     /// [`ManuallyDrop::into_inner`]: #method.into_inner
     #[stable(feature = "manually_drop", since = "1.20.0")]