test: fix indentation for -shuffle help text

test: fix indentation for -shuffle help text

The help text for the go test -shuffle flag is not indented like the
other flags. This patch brings it into alignment.

Fixes #49357

Change-Id: I3f18dc7cd84d5f23099262acf6e2fedccb11379c
Reviewed-on: https://go-review.googlesource.com/c/go/+/361395
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>