error-values: add issue link

Change-Id: I1d0cbcf65643f68759c071fba58cd7482695e332
Reviewed-on: https://go-review.googlesource.com/c/159498
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/design/29934-error-values.md b/design/29934-error-values.md
index c3fe5fc..b138f71 100644
--- a/design/29934-error-values.md
+++ b/design/29934-error-values.md
@@ -7,7 +7,7 @@
 
 Last updated: January 24, 2019
 
-Discussion at: TODO NEW ISSUE
+Discussion at: https://golang.org/issue/29934
 
 Past discussion at:
 - https://golang.org/design/go2draft-error-inspection