driver: add a space before +build in build tag comment

Add a space before a build tag comment so it corresponds to the format
documented at https://golang.org/pkg/go/build/.

Change-Id: Ic3225a76ca2ac1ce7de1dca7a1bbeaa438929a5a
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/193797
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/driver/driver_stub.go b/driver/driver_stub.go
index 31ff843..6f68af9 100644
--- a/driver/driver_stub.go
+++ b/driver/driver_stub.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//+build aix dragonfly freebsd netbsd openbsd solaris
+// +build aix dragonfly freebsd netbsd openbsd solaris
 
 package driver