Fix typo in buildall.sh

csharp/buildall.sh

@@ -6,7 +6,7 @@ NUNIT_CONSOLE=nunit-console
 
 # The rest you can leave intact
 CONFIG=Release
-KEYFILE=../keys/Google.Google.Protobuf.snk  # TODO(jtattermusch): signing!
+KEYFILE=../keys/Google.Protobuf.snk  # TODO(jtattermusch): signing!
 SRC=$(dirname $0)/src
 
 set -ex