Update file tree in README to point out pybind11

Signed-off-by: Addisu Z. Taddese <addisu@openrobotics.org>

README.md

@@ -63,7 +63,7 @@ gz-math
 ├── include/gz/math          Header files.
 ├── src                      Source files and unit tests.
 │   └── graph                Source files for the graph classes.
-│   └── python               SWIG Python interfaces.
+│   └── python_pybind11      Pybind11 Python interfaces.
 │   └── ruby                 SWIG Ruby interfaces.
 ├── eigen3                   Files for Eigen component.
 ├── test