README: Fix project tree

Author: sreea05

README.md

@@ -85,7 +85,8 @@ A **template** C++ library exposed to Python via [pybind11](https://github.com/p
     │   └── api.rst
     ├── src/
     │   └── core_lib/
-    │       └── __init__.py     # Re-exports from _core
+    │       ├── __init__.py     # Re-exports from _core
+    │       └── math.py
     └── tests/
         ├── unit/
         │   └── test_core_lib.py        # Unit tests (individual functions)