all: single space after period.

The tree's pretty inconsistent about single space vs double space
after a period in documentation. Make it consistently a single space,
per earlier decisions. This means contributors won't be confused by
misleading precedence.

This CL doesn't use go/doc to parse. It only addresses // comments.
It was generated with:

$ perl -i -npe 's,^(\s*// .+[a-z]\.)  +([A-Z]),$1 $2,' $(git grep -l -E '^\s*//(.+\.)  +([A-Z])')
$ go test go/doc -update

Change-Id: Iccdb99c37c797ef1f804a94b22ba5ee4b500c4f7
Reviewed-on: https://go-review.googlesource.com/20022
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Dave Day <djd@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/global_test.go

@@ -17,7 +17,7 @@ import (
 )
 
 // Make sure "hello world" does not link in all the
-// fmt.scanf routines.  See issue 6853.
+// fmt.scanf routines. See issue 6853.
 func TestScanfRemoval(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 
@@ -64,7 +64,7 @@ func main() {
 	}
 }
 
-// Make sure -S prints assembly code.  See issue 14515.
+// Make sure -S prints assembly code. See issue 14515.
 func TestDashS(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 
@@ -99,7 +99,7 @@ func main() {
 
 	patterns := []string{
 		// It is hard to look for actual instructions in an
-		// arch-independent way.  So we'll just look for
+		// arch-independent way. So we'll just look for
 		// pseudo-ops that are arch-independent.
 		"\tTEXT\t",
 		"\tFUNCDATA\t",