update readme.txt

Updated tested up to version to 5.1 in the readme.txt file.

Author: patilvikasj

readme.txt

@@ -3,7 +3,7 @@ Contributors: brainstormforce
 Donate link: https://www.paypal.me/BrainstormForce
 Tags: custom typekit fonts, theme custom fonts, unlimited typekit custom fonts
 Requires at least: 4.4
-Tested up to: 5.0
+Tested up to: 5.1
 Stable tag: 1.0.10
 License: GPLv2 or later
 License URI: http://www.gnu.org/licenses/gpl-2.0.html