docs(version-pinning): correct subpath guidance after smoke-test

Tested both flows in scratch modules:
- go get of a subpath (terratest/modules/terraform@v0.51.0) succeeds and
  resolves to the root module pin.
- go mod edit -require of the same subpath fails on tidy with
  "unknown revision modules/terraform/v0.51.0", which is what the
  customer hit in #1615.

Reframe the second section so the warning targets the actual foot-gun
(hand-written subpath requires) rather than claiming subpaths are always
broken.

Author: james00012

docs/_docs/01_getting-started/version-pinning.md

@@ -22,6 +22,6 @@ go mod tidy
 
 Replace `v0.56.0` with the tag you want from the [Releases page](https://github.com/gruntwork-io/terratest/releases). The same command upgrades, downgrades, or reverts the pin. Always commit `go.mod` and `go.sum` so the pin travels with your code; avoid `@latest` if you need reproducibility.
 
-## Use the root module path
+## Use `go get`, not subpath requires
 
-Pin `github.com/gruntwork-io/terratest`, not submodule paths like `github.com/gruntwork-io/terratest/modules/terraform`. Terratest publishes a single Go module at the repository root, so submodule-style versions (e.g. `modules/terraform/v0.51.0`) fail with `unknown revision`. Subpackage imports in your test code are fine; they all resolve through the root module pin.
+`go get github.com/gruntwork-io/terratest@<tag>` is enough; the pin applies to every subpackage your tests import. Avoid writing subpath requires by hand (e.g. `go mod edit -require github.com/gruntwork-io/terratest/modules/terraform@<tag>`); Go searches for a `modules/terraform/<tag>` tag and fails with `unknown revision`.