docs: add fully qualified example link

[NibiruHeisenberg]

Clicking on the example link in https://registry.terraform.io/modules/terraform-google-modules/vm/google/latest/submodules/mig leads to a 404 page.

I'm proposing a change to make the URL the same as the umig example.

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[comment-bot-dev]

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

• ⚠️check_generate_modules The modules need to be regenerated. Please run make_build.

Checking submodule's files generation
diff -r '--exclude=.terraform' '--exclude=.kitchen' '--exclude=.git' /workspace/modules/mig/README.md /tmp/tmp.56A35jgFq4/workspace/modules/mig/README.md
8c8
< See the [simple example](https://github.com/terraform-google-modules/terraform-google-vm/tree/master/examples/mig/simple) for a usage example.
---
> See the [simple example](../../examples/mig/simple) for a usage example.
Error: submodule's files generation has not been run, please run the
'make build' command and commit changes

[NibiruHeisenberg]

I ran make build locally but it just reset the URL to ../../examples/mig/simple. It doesn't affect the URL in the modules/umig/README.md file though.

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