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 f12de57..d9eaba8 100644
--- a/alts/src/main/java/io/grpc/alts/ComputeEngineChannelBuilder.java
+++ b/alts/src/main/java/io/grpc/alts/ComputeEngineChannelBuilder.java
@@ -39,7 +39,7 @@
  * using ALTS if applicable and using TLS as fallback.
  */
 public final class ComputeEngineChannelBuilder
-    extends ForwardingChannelBuilder<GoogleDefaultChannelBuilder> {
+    extends ForwardingChannelBuilder<ComputeEngineChannelBuilder> {
 
   private final NettyChannelBuilder delegate;