all: make copyright headers consistent with one space after period

Follows suit with https://go-review.googlesource.com/#/c/20111.

Generated by running
$ grep -R 'Go Authors.  All' * | cut -d":" -f1 | while read F;do perl -pi -e 's/Go
Authors.  All/Go Authors. All/g' $F;done

The code in cmd/internal/unvendor wasn't changed.

Fixes #15213

Change-Id: I4f235cee0a62ec435f9e8540a1ec08ae03b1a75f
Reviewed-on: https://go-review.googlesource.com/21819
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/bootstrap.bash b/src/bootstrap.bash
index 7947e84..da3dff4 100755
--- a/src/bootstrap.bash
+++ b/src/bootstrap.bash
@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# Copyright 2015 The Go Authors.  All rights reserved.
+# Copyright 2015 The Go Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.