fix typo in environment variable reference
@@ -6,7 +6,7 @@ Here are the environment variables CuPy uses.
+------------------------------------+----------------------------------------------------+
| ``CUPY_CACHE_DIR`` | Path to the directory to store kernel cache. |
-| | ``$(HOME)/.cupy.kernel_cache`` is used by default. |
+| | ``${HOME}/.cupy/kernel_cache`` is used by default. |
| | See :ref:`overview` for details. |
+------------------------------------+----------------------------------------------------+
| ``CUPY_CACHE_SAVE_CUDA_SOURCE`` | If set to 1, CUDA source file will be saved along |
