fix name, date and RFC PR link for rust-lang#391

Author: locks

text/0000-router-helpers.md renamed to text/0391-router-helpers.md

@@ -1,5 +1,5 @@
-- Start Date: (fill me in with today's date, YYYY-MM-DD)
-- RFC PR: (leave this empty)
+- Start Date: 2018-10-22
+- RFC PR: https://github.com/emberjs/rfcs/pull/391
 - Ember Issue: (leave this empty)
 
 # Router Helpers