Fix default cache directory to .pytest_cache

Author: nicoddemus

doc/en/reference.rst

@@ -891,7 +891,7 @@ passed multiple times. The expected format is ``name=value``. For example::
    .. versionadded:: 3.2
 
    Sets a directory where stores content of cache plugin. Default directory is
-   ``.cache`` which is created in :ref:`rootdir <rootdir>`. Directory may be
+   ``.pytest_cache`` which is created in :ref:`rootdir <rootdir>`. Directory may be
    relative or absolute path. If setting relative path, then directory is created
    relative to :ref:`rootdir <rootdir>`. Additionally path may contain environment
    variables, that will be expanded. For more information about cache plugin