Fix span in char documentation

Author: estebank

src/libstd/primitive_docs.rs

@@ -302,7 +302,7 @@ mod prim_never { }
 /// ```text
 /// error: character literal may only contain one codepoint: 'é'
 /// let c = 'é';
-///         ^^^^
+///         ^^^
 /// ```
 ///
 /// Another implication of the 4-byte fixed size of a `char` is that