_content/doc/install/source: mention GOAMD64
_content/doc/install/source: mention GOAMD64

Fixes golang/go#51704

Change-Id: I5f9efbbcd615ca7142311d8082fe1b36654df4eb
Reviewed-on: https://go-review.googlesource.com/c/website/+/393554
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@google.com>
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

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