Fix theme URI (#6707)

Fix theme URI to be https://github.com/automattic/themes/tree/trunk/blank-canvas-3

blank-canvas-3/style.css

@@ -1,6 +1,6 @@
 /*
 Theme Name: Blank Canvas
-Theme URI: https://github.com/automattic/themes/tree/trunk/blank-canvas
+Theme URI: https://github.com/automattic/themes/tree/trunk/blank-canvas-3
 Author: Automattic
 Author URI: https://automattic.com
 Description:Blank Canvas is a barebones starter theme, stripped off of content templates but only a footer and a header. With its minimal styling, Blank Canvas is a great theme starting fresh with your website.