Update README With New Branch Names

Author: oleibman

README.md

@@ -10,7 +10,7 @@
 PhpSpreadsheet is a library written in pure PHP and offers a set of classes that
 allow you to read and write various spreadsheet file formats such as Excel and LibreOffice Calc.
 
-This branch (release222) is maintained (for security and some bug fixes), but it is *not* the latest version of PhpSpreadsheet, and may therefore lack features and bug fixes found in the latest version.
+This branch (2.4.x) is maintained (for security and some bug fixes), but it is *not* the latest version of PhpSpreadsheet, and may therefore lack features and bug fixes found in the latest version.
 
 ## PHP Version Support