all: update 'go get' command in standard library README.vendor

The -m flag is removed in Go 1.13. -d should be used instead.

Change-Id: Ia53764748309f16cb231e5ac6770400a73804484
Reviewed-on: https://go-review.googlesource.com/c/go/+/191621


Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>