Clarify error phrase in sub_instant function

Uses the same wording as [`src/libstd/sys/windows/time.rs`][1].

1: 95e2bf253d/src/libstd/sys/windows/time.rs (L65)

src/libstd/sys/unix/time.rs

@@ -289,7 +289,7 @@ mod inner {
 
         pub fn sub_instant(&self, other: &Instant) -> Duration {
             self.t.sub_timespec(&other.t).unwrap_or_else(|_| {
-                panic!("other was greater than the current instant")
+                panic!("specified instant was later than self")
             })
         }