sys::unix::time.rs has spelling error in Instant::sub_duration

[krampenschiesser]

I was subtracting a duration from an instant and on travis/macos this fails with the following message:
overflow when adding duration to instant
This is a copy&paste/spelling error because I am using sub:

Current code in sys::unix::time.rs (sys::windows is correct):

        pub fn sub_duration(&self, other: &Duration) -> Instant {
            Instant {
                t: self.t.checked_sub(dur2intervals(other))
                       .expect("overflow when adding duration to instant"),
            }
        }

Should be

        pub fn sub_duration(&self, other: &Duration) -> Instant {
            Instant {
                t: self.t.checked_sub(dur2intervals(other))
                       .expect("overflow when subtracting duration from time"),
            }
        }

Meta

rustc --version --verbose:

rustc 1.18.0-nightly (252d3da8a 2017-04-22)
binary: rustc
commit-hash: 252d3da8a6c715ccafcf77d83b826f6fb899cfe5
commit-date: 2017-04-22
host: x86_64-unknown-linux-gnu
release: 1.18.0-nightly
LLVM version: 3.9