Blank Canvas Blocks: Update templateParts json (#3528)

blank-canvas-blocks/experimental-theme.json

@@ -1,4 +1,10 @@
 {
+	"templateParts": [
+		{
+			"name": "header",
+			"area": "header"
+		}
+	],
 	"settings": {
 		"defaults": {
 			"color": {