Update README for tflint config placeholder substitution

**Describe the bug**
A clear and concise description of what the bug is.
The README docs state that the tflint hook uses a `__GIT_DIR__` placeholder to be substituted with the actual Git repository root. However, the script instead looks for a `__GIT_ROOT__` placeholder.

**To Reproduce**
Steps to reproduce the behavior including the relevant Terraform/Terragrunt/Packer version number and any code snippets and module inputs you used.

```hcl
// paste code snippets here
```

**Expected behavior**
A clear and concise description of what you expected to happen.

**Nice to have**
- [ ] Terminal output
- [ ] Screenshots

**Additional context**
Add any other context about the problem here.