[boringssl] Updating rolling instructions

Update the instructions on the bindgen dependency as people
frequently trip over it.

Change-Id: I49bbef2f95884a15034cfc4bae9b1838cf6997df
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/boringssl/+/615240
Fuchsia-Auto-Submit: Ali Zhang <alizhang@google.com>
Reviewed-by: Allison Pearce <ampearce@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>