correct typo in `.hbs` file extension
add `.hbs` file extension to make Handlebars process markdown files as templates
use files array format, add `ext: ''` to make assemble drop the `.hbs` extension and leave only `.md` on dest files. Also add `cwd: 'src'` and adjust src path, since dest files aren't flattened and we don't want the "src" folder to be copied to the dest folder.