Fix checkstyle

Author: szetszwo

ratis-grpc/src/main/java/org/apache/ratis/grpc/GrpcFactory.java

@@ -73,7 +73,7 @@ static boolean checkPooledByteBufAllocatorUseCacheForAllThreads(Consumer<String>
   static final BiFunction<GrpcTlsConfig, SslContext, SslContext> BUILD_SSL_CONTEXT_FOR_CLIENT
       = (tlsConf, defaultContext) -> tlsConf == null ? defaultContext : GrpcUtil.buildSslContextForClient(tlsConf);
 
-  static class SslContexts {
+  static final class SslContexts {
     private final SslContext adminSslContext;
     private final SslContext clientSslContext;
     private final SslContext serverSslContext;