Update commits since badge to beta8

Author: nickclark2016

README.md

@@ -5,7 +5,7 @@
 <p align="center">
     <img src="https://img.shields.io/github/release/premake/premake-core/all.svg" alt="Latest release" />
     <img src="https://img.shields.io/github/release-date-pre/premake/premake-core.svg" alt="Release date" />
-    <img src="https://img.shields.io/github/commits-since/premake/premake-core/v5.0.0-beta6.svg" alt="Commits" />
+    <img src="https://img.shields.io/github/commits-since/premake/premake-core/v5.0.0-beta8.svg" alt="Commits" />
     <a href="https://opensource.org/licenses/BSD-3-Clause" target="_blank">
         <img src="https://img.shields.io/github/license/premake/premake-core" alt="BSD 3-Clause" />
     </a>