Fixed typo in the comment of Date.init(timeIntervalSinceReferenceDate:) (#435)

Author: KentaKudo

Foundation/Date.swift

@@ -49,7 +49,7 @@ public struct Date : ReferenceConvertible, Comparable, Equatable, CustomStringCo
         self.init(timeIntervalSinceReferenceDate: date.timeIntervalSinceReferenceDate + timeInterval)
     }
 
-    /// Returns an `Date` initialized relative to 00:00:00 UTC on 1 January 2001 by a given number of seconds.
+    /// Returns a `Date` initialized relative to 00:00:00 UTC on 1 January 2001 by a given number of seconds.
     public init(timeIntervalSinceReferenceDate ti: TimeInterval) {
         _time = ti
     }