design/16339-alias-decls.md: fix quoting for syntax production

For golang/go#16339.

Change-Id: I107d3ceea6a32aa44c712f1ca035f282a19de149
Reviewed-on: https://go-review.googlesource.com/25041
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/design/16339-alias-decls.md b/design/16339-alias-decls.md
index 8e29ee7..b57f75b 100644
--- a/design/16339-alias-decls.md
+++ b/design/16339-alias-decls.md
@@ -151,7 +151,9 @@
 
 With that, a general alias specification is of the form:
 
+```
 AliasSpec = identifier "=>" PackageName "." identifier .
+```
 
 Per the discussion at https://golang.org/issue/16339, and based on feedback
 from adonovan@golang, to avoid abuse, alias declarations may refer to imported