Update 2.12.13.md

Author: lrytz

releases/2.12.13.md

@@ -1,6 +1,6 @@
 Scala 2.12.13:
 
-* Backports configurable warnings and errors from 2.13.x.  // TODO link to blog post
+* Backports configurable warnings and errors from 2.13.x. // TODO: merge https://github.com/scala/scala-lang/pull/1184 and add a link to the post
 * Improves pattern matching exhaustivity warnings for patterns involving Tuples.
 * Improves performance of building `immutable.{TreeMap,TreeSet}` by using mutation within
   the builder.