docs(readme): point Stability section at MIGRATION.md

PR #1803 places the migration guide at root-level MIGRATION.md
rather than the docs site. Update the README link accordingly so
readers reach the actual content.

Linear: OSS-3393

Author: james00012

README.md

@@ -36,8 +36,7 @@ Starting with v1.0.0, Terratest follows [semantic versioning](https://semver.org
 only happen in major releases (e.g. v2.0.0).
 
 Symbols renamed or replaced in v1 are kept with `// Deprecated:` annotations pointing at the new name; removals happen
-in v2. Migrating from v0.x: see the [v1 release notes and migration guide](https://terratest.gruntwork.io/docs/) on the
-Terratest docs site.
+in v2. Migrating from v0.x: see [`MIGRATION.md`](./MIGRATION.md).
 
 ## More info