html/template: use strings.Builder

html/template: use strings.Builder

Change-Id: Iab6ef8c6c74c09e8358f7d7088a82089725479ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/428260
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>