Point UNIX_EPOCH to associated constant in SystemTime docs

Author: coolreader18

std/src/time.rs

@@ -245,6 +245,7 @@ pub struct Instant(time::Instant);
 /// > structure cannot represent the new point in time.
 ///
 /// [`add`]: SystemTime::add
+/// [`UNIX_EPOCH`]: SystemTime::UNIX_EPOCH
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[stable(feature = "time2", since = "1.8.0")]
 pub struct SystemTime(time::SystemTime);