Updated docs that reference how to base64 encode binary files

[melinath]

file only opens utf8-encoded files https://www.terraform.io/docs/language/functions/filebase64.html Ran into this while working on hashicorp/terraform-provider-google#10164

[megan07]

lgtm!

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 2 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 2 files changed, 2 insertions(+), 2 deletions(-))