docs: fix spelling of dismissible (#3689)

Because the spelling of the "dismissible" arg changed in the dialog
component, the spelling needed to be updated in popover as well.

Author: marissahuysentruyt

components/popover/stories/popover.stories.js

@@ -246,7 +246,7 @@ DialogStyle.args = {
 		(passthroughs, context) => Dialog({
 			showModal: false,
 			size: "s",
-			isDismissable: false,
+			isDismissible: false,
 			heading: "Example heading",
 			hasFooter: false,
 			footer: [""],

components/popover/stories/popover.test.js

@@ -139,7 +139,7 @@ export const PopoverGroup = Variants({
 				(passthroughs, context) => Dialog({
 					showModal: false,
 					size: "s",
-					isDismissable: false,
+					isDismissible: false,
 					heading: "Example heading",
 					hasFooter: false,
 					footer: [""],