Fix comment.

The underlying method saturates. This comment was likely written with an old underlying implementation.

Signed-off-by: Kevin Cox <[email protected]>

neptun/src/sleepyinstant/mod.rs

@@ -52,7 +52,7 @@ impl Instant {
     ///
     /// # Panics
     ///
-    /// panics when `earlier` was later than `self`.
+    /// Returns `Duration::ZERO` when `earlier` was later than `self`.
     pub fn duration_since(&self, earlier: Instant) -> Duration {
         self.t.duration_since(earlier.t)
     }