fix doc for pointer's validity in zst operation

Author: DiuDiu777

library/core/src/ptr/mod.rs

@@ -15,7 +15,7 @@
 //! The precise rules for validity are not determined yet. The guarantees that are
 //! provided at this point are very minimal:
 //!
-//! * For operations of [size zero][zst], *every* pointer is valid, including the [null] pointer.
+//! * For operations of [size zero][zst], *every* pointer is valid, except the [null] pointer.
 //!   The following points are only concerned with non-zero-sized accesses.
 //! * A [null] pointer is *never* valid.
 //! * For a pointer to be valid, it is necessary, but not always sufficient, that the pointer be