56345-structured-logging.md: add issue number

Change-Id: I7629efed8d63a1973b7c3d4500d2afc8a096df32
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/446870
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
diff --git a/design/56345-structured-logging.md b/design/56345-structured-logging.md
index a92d2a0..93f6e6f 100644
--- a/design/56345-structured-logging.md
+++ b/design/56345-structured-logging.md
@@ -4,7 +4,7 @@
 
 Date: 2022-10-19
 
-Issue: https://go.dev/issue/NNNNN
+Issue: https://go.dev/issue/56345
 
 Discussion: https://github.com/golang/go/discussions/54763