update readme to reflect tested up to 6.9 (#58)

Author: aurooba

readme.txt

@@ -2,7 +2,7 @@
 Contributors:      aurooba, cr0ybot
 Tags:              block, list, nesting, repeater, superlist
 Requires at least: 6.3
-Tested up to:      6.7.2
+Tested up to:      6.9
 Requires PHP:      7.0
 Stable tag:        0.1.4
 License:           GPL-2.0-or-later