Rollup merge of rust-lang#55247 - peterjoel:peterjoel-prim-char-doc-example, r=joshtriplett

Clarified code example in char primitive doc

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 elements in the slice by the item size. This change demonstrates the idea more straightforwardly, without needing a calculation, by just comparing the size of the slices.

Author: pietroalbini

src/libstd/primitive_docs.rs

@@ -323,8 +323,8 @@ 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 { }