Fix confusing error message for sub_instant

When subtracting an Instant from another, the function will panick when `RHS > self`, but the error message confusingly displays a different error:

```rust
let i = Instant::now();
let other = Instant::now();
if other > i {
    println!("{:?}", i - other);
}
```
This results in a panic:
```
thread 'test_instant' panicked at 'other was less than the current instant', libstd/sys/unix/time.rs:292:17
```

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstd/sys/unix/time.rs b/src/libstd/sys/unix/time.rs
index 83127935909..f7459cb55d5 100644
--- a/src/libstd/sys/unix/time.rs
+++ b/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 less than the current instant")
+                panic!("other was greater than the current instant")
             })
         }