notmuch-hello: Fix ability to isearch to a saved-search and press RET

After isearching for an entire saved-search name, the point will be
immediately after that name in the buffer. Before commit
c9ba61bebef7733c4bf12adf94222e57621fdcf6 the space right after the
name was part of the widget so the user could press RET right after
the isearch to activate the saved search.

The above commit broke that functionality. Restore it by including a
single space after each name as part of the widget.