blog: use serial comma in long author lists

This is the Go blog house style for the rest of the text.
We just missed the author list, presumably because it
was less code (only two entries still has no comma).

Change-Id: I8bb1ab582d6e300d521ac471736ee8ab6e0f61ec
Reviewed-on: https://go-review.googlesource.com/c/tools/+/225517
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

blog/blog.go

@@ -159,6 +159,9 @@ func authors(authors []present.Author) string {
 	for i, a := range authors {
 		if i > 0 {
 			if i == last {
+				if len(authors) > 2 {
+					b.WriteString(",")
+				}
 				b.WriteString(" and ")
 			} else {
 				b.WriteString(", ")