Update reference/src/layout/unions.md

Co-Authored-By: Ralf Jung <post@ralfj.de>

Author: gnzlbg

reference/src/layout/unions.md

@@ -105,7 +105,7 @@ assert_eq!(align_of::<Zst2>(), 16);
 the layout of `U1` is **unspecified** because:
 
 * `Zst2` is not a [1-ZST], and
-* `SomeOtherStruct` has an unspecified layout and could contain padding bits.
+* `SomeOtherStruct` has an unspecified layout and could contain padding bytes.
 
 ### C-compatible layout ("repr C")