remove caption also on backSpace

wax-prosemirror-plugins/src/images/captionPlugin.js

@@ -85,7 +85,7 @@ const captionPlugin = key =>
         },
         keyup(view, e) {
           // delete caption if figure is deleted
-          if (e.key === 'Delete') {
+          if (e.key === 'Delete' || e.code === 'Backspace') {
             const figcap = document.getElementsByTagName('figcaption');
             const figcapLength = figcap.length;