vec::as_mut_slice(): use lowercase "isize" in safety comment

To match other references to that type

Author: DanielEScherzer

library/alloc/src/vec/mod.rs

@@ -1853,7 +1853,7 @@ impl<T, A: Allocator> Vec<T, A> {
         // SAFETY: `slice::from_raw_parts_mut` requires pointee is a contiguous, aligned buffer of
         // size `len` containing properly-initialized `T`s. Data must not be accessed through any
         // other pointer for the returned lifetime. Further, `len * size_of::<T>` <=
-        // `ISIZE::MAX` and allocation does not "wrap" through overflowing memory addresses.
+        // `isize::MAX` and allocation does not "wrap" through overflowing memory addresses.
         //
         // * Vec API guarantees that self.buf:
         //      * contains only properly-initialized items within 0..len