Update README.md

fix version number

Author: kerlerm

README.md

@@ -22,7 +22,7 @@ Build commands for the console:
     cd openwrt
     git reset --hard 6fc02f2a45e151ce16677d6131251af86ab4fc06
 
-    git clone -b v2.3.0 https://github.com/ffulm/firmware.git
+    git clone -b v2.3.1 https://github.com/ffulm/firmware.git
     cp -rf firmware/files firmware/package firmware/feeds.conf .
 
     ./scripts/feeds update -a