javadoc: fix broken links in javadoc

diff --git a/core/src/main/java/io/grpc/internal/AbstractManagedChannelImplBuilder.java b/core/src/main/java/io/grpc/internal/AbstractManagedChannelImplBuilder.java
index 35e1950..fbe2dde 100644
--- a/core/src/main/java/io/grpc/internal/AbstractManagedChannelImplBuilder.java
+++ b/core/src/main/java/io/grpc/internal/AbstractManagedChannelImplBuilder.java
@@ -42,6 +42,7 @@
 import com.google.instrumentation.trace.Tracing;
 import io.grpc.Attributes;
 import io.grpc.ClientInterceptor;
+import io.grpc.ClientStreamTracer;
 import io.grpc.CompressorRegistry;
 import io.grpc.DecompressorRegistry;
 import io.grpc.EquivalentAddressGroup;