Rollup merge of rust-lang#41518 - tbu-:pr_fix_cp_error, r=sfackler

Fix a copy-paste error in `Instant::sub_duration`

Fixes rust-lang#41514.

Author: Ariel Ben-Yehuda

src/libstd/sys/unix/time.rs

@@ -157,7 +157,7 @@ mod inner {
         pub fn sub_duration(&self, other: &Duration) -> Instant {
             Instant {
                 t: self.t.checked_sub(dur2intervals(other))
-                       .expect("overflow when adding duration to instant"),
+                       .expect("overflow when subtracting duration from instant"),
             }
         }
     }