alts: fix ComputeEngineChannelBuilder class signature

diff --git a/alts/src/main/java/io/grpc/alts/ComputeEngineChannelBuilder.java b/alts/src/main/java/io/grpc/alts/ComputeEngineChannelBuilder.java
index af3376f..e71b2c9 100644
--- a/alts/src/main/java/io/grpc/alts/ComputeEngineChannelBuilder.java
+++ b/alts/src/main/java/io/grpc/alts/ComputeEngineChannelBuilder.java
@@ -44,7 +44,7 @@
  * using ALTS if applicable and using TLS as fallback.
  */
 public final class ComputeEngineChannelBuilder
-    extends ForwardingChannelBuilder<GoogleDefaultChannelBuilder> {
+    extends ForwardingChannelBuilder<ComputeEngineChannelBuilder> {
 
   private final NettyChannelBuilder delegate;
   private GoogleDefaultProtocolNegotiator negotiatorForTest;