netty: Fix Javadoc reference to Channelz.Security

This fixes the warning:
`Tag @link: reference not found: Channelz.Security`

Javadoc `@link` is simplistic in its processing of '.' and thinks if a
dot exists it means it is part of the package name. You're forced to use
the full name of nested classes.

netty/src/main/java/io/grpc/netty/GrpcHttp2ConnectionHandler.java

@@ -46,7 +46,7 @@ public abstract class GrpcHttp2ConnectionHandler extends Http2ConnectionHandler
 
   /**
    * Same as {@link #handleProtocolNegotiationCompleted(Attributes, Channelz.Security)}
-   * but with no {@link Channelz.Security}.
+   * but with no {@link io.grpc.internal.Channelz.Security}.
    *
    * @deprecated Use the two argument method instead.
    */