js: isso: Fix class selector for preview (#840)
js: isso: Fix class selector for preview (#840)

The selector also needs the `isso-` prefix, else the
DOM selection fails and the subsequent code gets a null obj,
which then fails.

This was overlooked in 85f3954c80bc68feb0de23c186d06c716016faae
"js: app/isso: Prefix localstorage keys w/ isso-"

Fixes https://github.com/posativ/isso/issues/839
posativ/isso,posativ/isso,posativ/isso,posativ/isso