Update README link from Terraform to OpenTofu

HashiCorp relicensed Terraform from the MPL (Mozilla Public License) to
the non-open-source BSL (Business Source License). Terraform 1.5 was the
final release to be licensed under MPL.

OpenTofu is an open-source alternative to HashiCorp Terraform. It was
initially called OpenTF. This commit will update a README link to point
to the OpenTofu docs (https://opentofu.org/docs/intro/use-cases/).

https://formulae.brew.sh/formula/opentofu
https://formulae.brew.sh/formula/terraform
https://github.com/opentofu/opentofu
https://opentofu.org/
https://www.hashicorp.com/blog/hashicorp-adopts-business-source-license
https://blog.gruntwork.io/the-future-of-terraform-must-be-open-ab0b9ba65bca

README.md

@@ -41,7 +41,7 @@ This repo contains dotfiles, which are application configuration and settings fi
 
 ### Why
 
-- **Make developer environments automated and disposable**. [Disposability](https://12factor.net/disposability) is an important concept in [infrastructure-as-code DevOps](https://www.terraform.io/intro/use-cases#disposable-environments), [serverless computing](https://www.cloudflare.com/learning/serverless/what-is-serverless/), [CI/CD](https://docs.github.com/en/actions/learn-github-actions/understanding-github-actions), and more recently, [in-browser development environments](https://docs.github.com/en/codespaces/overview). Why aren't developers applying automation and disposability to their own computers? With an automated disposable developer environment, setup of a new machine is fast and easy. This approach is also liberating - I can purchase a new computer (or wipe an existing one), run _bootstrap.sh_, and be up and running again in no time.
+- **Make developer environments automated and disposable**. [Disposability](https://12factor.net/disposability) is an important concept in [infrastructure-as-code DevOps](https://opentofu.org/docs/intro/use-cases/), [serverless computing](https://www.cloudflare.com/learning/serverless/what-is-serverless/), [CI/CD](https://docs.github.com/en/actions/learn-github-actions/understanding-github-actions), and more recently, [in-browser development environments](https://docs.github.com/en/codespaces/overview). Why aren't developers applying automation and disposability to their own computers? With an automated disposable developer environment, setup of a new machine is fast and easy. This approach is also liberating - I can purchase a new computer (or wipe an existing one), run _bootstrap.sh_, and be up and running again in no time.
 - **Know when and why settings change**. I not only know what tools and settings I'm using, but when and why I chose the tools and settings. This has been particularly important for VSCode, because settings change (and [break](https://github.com/microsoft/vscode/labels/bug)) frequently, and it helps to record troubleshooting info in the Git log.
 - **Learn new skills**. I learn skills, like shell scripting, that are useful and don't go out of date quickly. I wouldn't know shell as well if I didn't work on my developer environment. I learn these skills by tinkering a little bit at a time, in an unstructured way. It's time I might not otherwise be writing code.