Changed positive modifier for font compatibility from the letter x to a ...

[williamdodson]

Minor (mostly cosmetic) change: changed the positive modifier symbol for when a font is present on a given platform from "x" to a "+" plus sign. Visually-impaired users cannot see the green background on table cells where a particular font is available and the "x" may be confusing; i.e. is this font available or not? The plus sign is really the only universal positive symbol available (check marks in some countries denote false).