Rollup merge of rust-lang#61715 - RalfJung:test-ascii-lowercase, r=varkor

make sure make_ascii_lowercase actually leaves upper-case non-ASCII characters alone

Cc rust-lang#61677 @napen123

Author: Centril

src/libcore/str/mod.rs

@@ -4000,11 +4000,11 @@ impl str {
     /// # Examples
     ///
     /// ```
-    /// let mut s = String::from("Grüße, Jürgen ❤");
+    /// let mut s = String::from("GRÜßE, JÜRGEN ❤");
     ///
     /// s.make_ascii_lowercase();
     ///
-    /// assert_eq!("grüße, jürgen ❤", s);
+    /// assert_eq!("grÜße, jÜrgen ❤", s);
     /// ```
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
     pub fn make_ascii_lowercase(&mut self) {