Rollup merge of #76162 - abrausch:documentation-fix-duration_since, r=jyn514

Make duration_since documentation more clear

library/std/src/time.rs

@@ -460,11 +460,12 @@ impl SystemTime {
     ///
     /// # Examples
     ///
-    /// ```
+    /// ```no_run
     /// use std::time::SystemTime;
     ///
     /// let sys_time = SystemTime::now();
-    /// let difference = sys_time.duration_since(sys_time)
+    /// let new_sys_time = SystemTime::now();
+    /// let difference = new_sys_time.duration_since(sys_time)
     ///     .expect("Clock may have gone backwards");
     /// println!("{:?}", difference);
     /// ```