Clarify error phrase in sub_instant function

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

1: https://github.com/avdv/rust/blob/95e2bf253d864c5e14ad000ffa2040ce85916056/src/libstd/sys/windows/time.rs#L65

src/libstd/sys/redox/time.rs

@@ -144,7 +144,7 @@ impl Instant {
 
     pub fn sub_instant(&self, other: &Instant) -> Duration {
         self.t.sub_timespec(&other.t).unwrap_or_else(|_| {
-            panic!("other was less than the current instant")
+            panic!("specified instant was later than self")
         })
     }
 

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")
             })
         }