only add aria-modal when this.options.modal is set to true

suggestion by @mgol

Co-authored-by: Michał Gołębiowski-Owczarek <m.goleb@gmail.com>

Author: rpkoller

ui/widgets/dialog.js

@@ -348,7 +348,7 @@ $.widget( "ui.dialog", {
 				// Setting tabIndex makes the div focusable
 				tabIndex: -1,
 				role: "dialog",
-				"aria-modal": "true"
+				"aria-modal": this.options.modal ? "true" : null
 			} )
 			.appendTo( this._appendTo() );