dir/dircache: remove + from comment about build constraint

At some point vet started looking harder for build constraints, and that
broke our build because the "+build" string in the comment about the
"+build" constraint made it think we had a bad constraint.

Thanks to rsc for figuring this out. I doubt I ever would have.

Change-Id: I0ecb448c7ccf94ae8cfd25e52a4f0433b5870082
Reviewed-on: https://upspin-review.googlesource.com/c/upspin/+/19780
Reviewed-by: David Presotto <presotto@gmail.com>
diff --git a/dir/dircache/dump.go b/dir/dircache/dump.go
index 651c67c..b057db7 100644
--- a/dir/dircache/dump.go
+++ b/dir/dircache/dump.go
@@ -2,10 +2,11 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// This is meant for debugging. To include in the library, remove the +build line
-// and then go install.
+// This file is meant for debugging. To include in the library, remove the build
+// constraints (one for each historical format) and then go install.
 
-//+build ignore
+//go:build ignore
+// +build ignore
 
 package dircache