[docs] Change docs to use `fx serve` instead of `fx serve-updates`

Bug: 127446
Change-Id: I093ae8a43fb41aed27116279974875cccc5d2f4e
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/858585
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Adam Perry <adamperry@google.com>
Fuchsia-Auto-Submit: Erick Tryzelaar <etryzelaar@google.com>
Reviewed-by: Casey Dahlin <sadmac@google.com>
Reviewed-by: Sen Jiang <senj@google.com>