pybamm-team#3049 clarify extension language

CONTRIBUTING.md

@@ -413,7 +413,7 @@ wherever code is called that uses that citation (for example, in functions or in
 
 ### Installation
 
-Installation of PyBaMM and its dependencies is handled via [pip](https://pip.pypa.io/en/stable/) and [setuptools](http://setuptools.readthedocs.io/). It uses `CMake` to compile C extensions using [`pybind11`](https://pybind11.readthedocs.io/en/stable/) and [`casadi`](https://web.casadi.org/) (non-Windows). The installation process is described in detail in the [source installation](https://docs.pybamm.org/en/latest/source/user_guide/installation/install-from-source.html) page and is configured through the `CMakeLists.txt` file.
+Installation of PyBaMM and its dependencies is handled via [pip](https://pip.pypa.io/en/stable/) and [setuptools](http://setuptools.readthedocs.io/). It uses `CMake` to compile C++ extensions using [`pybind11`](https://pybind11.readthedocs.io/en/stable/) and [`casadi`](https://web.casadi.org/) (non-Windows). The installation process is described in detail in the [source installation](https://docs.pybamm.org/en/latest/source/user_guide/installation/install-from-source.html) page and is configured through the `CMakeLists.txt` file.
 
 Configuration files: