content: use gosumcheck command from x/mod subrepo

The gosumcheck command was copied out of x/exp subrepo into the new
x/mod subrepo in CL 176639. It's already being used on the index
page of https://proxy.golang.org. Use it in the blog post too.

Change-Id: I0d21c7d876baabf8b16d999c956512b62c47532f
Reviewed-on: https://go-review.googlesource.com/c/blog/+/192419
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/content/module-mirror-launch.article b/content/module-mirror-launch.article
index 58b811a..391fa16 100644
--- a/content/module-mirror-launch.article
+++ b/content/module-mirror-launch.article
@@ -114,10 +114,10 @@
 
 If you are using Go 1.12 or earlier, you can manually check a `go.sum` file
 against the checksum database with
-[[https://godoc.org/golang.org/x/exp/sumdb/gosumcheck][gosumcheck]]:
+[[https://godoc.org/golang.org/x/mod/gosumcheck][gosumcheck]]:
 
-    go get golang.org/x/exp/sumdb/gosumcheck
-    gosumcheck /path/to/go.sum
+    $ go get golang.org/x/mod/gosumcheck
+    $ gosumcheck /path/to/go.sum
 
 In addition to verification done by the `go` command, third-party
 auditors can hold the checksum database accountable by iterating over the log