Fix cast in performance API example (#2634)

Rust 1.45 defines (f64 as u32) as a saturating cast.
The millis was incorrectly always 295 (u32::MAX % 1000).

Author: andy-k

examples/performance/src/lib.rs

@@ -31,6 +31,6 @@ pub fn run() {
 
 fn perf_to_system(amt: f64) -> SystemTime {
     let secs = (amt as u64) / 1_000;
-    let nanos = ((amt as u32) % 1_000) * 1_000_000;
+    let nanos = (((amt as u64) % 1_000) as u32) * 1_000_000;
     UNIX_EPOCH + Duration::new(secs, nanos)
 }