Example -> Examples

This brings comments in line with https://github.com/rust-lang/rfcs/blob/master/text/0505-api-comment-conventions.md#using-markdown

src/libcore/str/mod.rs

@@ -1183,7 +1183,7 @@ mod traits {
     /// Panics when `begin` and `end` do not point to valid characters
     /// or point beyond the last character of the string.
     ///
-    /// # Example
+    /// # Examples
     ///
     /// ```rust
     /// let s = "Löwe 老虎 Léopard";