remove `credentials` var from readme

[sethidden]

it was removed in https://github.com/terraform-google-modules/terraform-google-cloud-datastore/commit/e8ae3a57f69f38af9ae26821269a61e6973d1018#diff-05b5a57c136b6ff596500bcbfdcff145ef6cddea2a0e86d184d9daa9a65a288e

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[comment-bot-dev]

@sethidden Thanks for the PR! 🚀 Unfortunately it looks like some of our CI checks failed. See the Contributing Guide for details.

• ⚠️check_tflint Failed TFlint check. More details below.

  Checking for tflint
  Working in . ...
  2 issue(s) found:
  Warning: Missing version constraint for provider "local" in "required_providers" (terraform_required_providers)
  on main.tf line 22:
  22: resource "local_file" "cloud-datastore-index-file" {
  Reference: https://github.com/terraform-linters/tflint-ruleset-terraform/blob/v0.1.1/docs/rules/terraform_required_providers.md
  Warning: Missing version constraint for provider "null" in "required_providers" (terraform_required_providers)
  on main.tf line 27:
  27: resource "null_resource" "cloud-datastore-indices" {
  Reference: https://github.com/terraform-linters/tflint-ruleset-terraform/blob/v0.1.1/docs/rules/terraform_required_providers.md
  tflint failed . 
  Working in ./examples/simple_example ...
  tflint passed ./examples/simple_example 
  Skipping ./test/fixtures/simple_example
  Skipping ./test/setup