Replace cidr_block with ipv4_cidr_block in README

[MajinBuuOnSecurity]

Solves #121, see https://github.com/cloudposse/terraform-aws-dynamic-subnets#usage for further evidence.

[joe-niland]

Hi @MajinBuuOnSecurity

Thanks for your PR.
To make this change, please update README.yaml then run:

make init
make readme

[hans-d]

/terratest

[hans-d]

From the top of the READme file involved

  ** DO NOT EDIT THIS FILE
  **
  ** This file was automatically generated by the `cloudposse/build-harness`.
  ** 1) Make all changes to `README.yaml`
  ** 2) Run `make init` (you only need to do this once)
  ** 3) Run`make readme` to rebuild this file.

@MajinBuuOnSecurity Can you apply the fix to the right file and execute the rebuild?

[mergify]

Thanks @MajinBuuOnSecurity for creating this pull request!

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