src/timeunits.rs: fix minor typo

Signed-off-by: Guillaume W. Bres <guillaume.bressaix@gmail.com>

src/timeunits.rs

@@ -33,7 +33,7 @@ pub enum Unit {
  Minute,
  Hour,
  Day,
- /// 36525 days, it the number of days per century in the Julian calendar
+ /// 36525 days, is the number of days per century in the Julian calendar
  Century,
 }