Point schema/README at the underscored generator output path

No-Verification-Needed: doc-only fix

Author: maxisbey

schema/README.md

@@ -3,8 +3,8 @@
 JSON Schema files for each protocol version the SDK has a wire-shape surface
 package for, vendored from the [spec repository] at the commit recorded in
 `PINNED.json`. `scripts/gen_surface_types.py` reads these to regenerate
-`src/mcp-types/mcp_types/v<version>/__init__.py`; CI runs the generator with
-`--check`.
+`src/mcp-types/mcp_types/_v<version>/__init__.py` (underscore-private: internal
+validators, not public API); CI runs the generator with `--check`.
 
 To bump: drop the new `schema.json` here as `<protocol-version>.json`, update
 the matching entry in `PINNED.json` (commit + sha256), and run