doc/go1.16: fix closing brace in .Export format

A parenthesis of go list "-f" flag format is double curly braces.

Change-Id: Ifd38e0b0ae3c46272a4acd65584818228168b7c6
GitHub-Last-Rev: b46030492b5caf18fe127621fdf92cbec4c03ad5
GitHub-Pull-Request: golang/go#43924
Reviewed-on: https://go-review.googlesource.com/c/go/+/286752


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>