Fix start date in 1419

Author: xzmeng

text/1419-slice-copy.md

@@ -1,5 +1,5 @@
 - Feature Name: slice\_copy\_from
-- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- Start Date: 2015-12-20
 - RFC PR: [rust-lang/rfcs#1419](https://github.com/rust-lang/rfcs/pull/1419)
 - Rust Issue: [rust-lang/rust#31755](https://github.com/rust-lang/rust/issues/31755)