_content/doc/modules/layout: fix typo in example file name

Change-Id: Ib8d73e5a636b36cc468d1d8b23bef660f8740f7d
GitHub-Last-Rev: a357c1c9ce
GitHub-Pull-Request: golang/website#251
Reviewed-on: https://go-review.googlesource.com/c/website/+/529736
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Eli Bendersky <eliben@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

_content/doc/modules/layout.md

@@ -55,7 +55,7 @@ project-root-directory/
   modname.go
   modname_test.go
   auth.go
-  auto_test.go
+  auth_test.go
   hash.go
   hash_test.go
 ```