_context/doc/modules: add GOPRIVATE to managing-dependencies
_context/doc/modules: add GOPRIVATE to managing-dependencies

Fixes golang/go#44240

Change-Id: I86d07db963fb0cebd5e6560422a2500d9b0d5164
Reviewed-on: https://go-review.googlesource.com/c/website/+/348990
Reviewed-by: jiahua wang <3c642a1f58b0bafd1f2b73c3c695b75071644a5d@gmail.com>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Steve Traut <af4cad49c6f706d4616eefecc2c596e86c262fed@google.com>
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website