cmd/go/internal/modload: fix an apparent typo in the AutoRoot comment

Updates #40276

Change-Id: Ic192d51f9f0306e5c206c550ef02f6d4495d0851
Reviewed-on: https://go-review.googlesource.com/c/go/+/332569
Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

Author: Bryan C. Mills

src/cmd/go/internal/modload/init.go

@@ -71,7 +71,7 @@ type Root int
 const (
 	// AutoRoot is the default for most commands. modload.Init will look for
 	// a go.mod file in the current directory or any parent. If none is found,
-	// modules may be disabled (GO111MODULE=on) or commands may run in a
+	// modules may be disabled (GO111MODULE=auto) or commands may run in a
 	// limited module mode.
 	AutoRoot Root = iota