fix date format in codegen.sh

codegen.sh

@@ -5,7 +5,7 @@
 #
 # It assumes the installation of i) Google proto buffer compiler at
 # https://github.com/google/protobuf (after v2.6.1) and ii) the Go codegen
-# plugin at https://github.com/golang/protobuf (after 2/19/2015). If you have
+# plugin at https://github.com/golang/protobuf (after 2015-02-20). If you have
 # not, please install them first.
 #
 # We recommend running this script at $GOPATH or $GOPATH/src.