Clarified code example

The example was not as clear as it could be because it was making an assumption about the structure of the data in order to multiply the number of collection elements by the item size. This change demonstrates the idea more straightforwardly, without the calculation.

src/libstd/primitive_docs.rs

@@ -322,8 +322,8 @@ mod prim_never { }
 /// let s = String::from("love: ❤️");
 /// let v: Vec<char> = s.chars().collect();
 ///
-/// assert_eq!(12, s.len() * std::mem::size_of::<u8>());
-/// assert_eq!(32, v.len() * std::mem::size_of::<char>());
+/// assert_eq!(12, std::mem::size_of_val(&s[..]));
+/// assert_eq!(32, std::mem::size_of_val(&v[..]));
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_char { }