Fix theme directory mapping

[dustin-jw]

The theme check plugin can be used to validate themes against a test suite to report any issues with the theme.

This warning about the theme directory is reported:

WARNING: Your theme appears to be in the wrong directory for the theme name. The directory name must match the slug of the theme. This theme's correct slug and text-domain is sparkpress. (If this is a child theme, you can ignore this error.)

We should update the volume mapping so the theme ends up in wp-content/themes/sparkpress instead of wp-content/themes/sparkpress-theme.