http: fix javadoc link

https://codereview.appspot.com/7796044/

Author: Yaniv Inbar

google-http-client/src/main/java/com/google/api/client/util/ExponentialBackOff.java

@@ -14,8 +14,6 @@
 
 package com.google.api.client.util;
 
-import com.google.api.client.http.BackOffPolicy;
-
 import java.io.IOException;
 
 /**
@@ -66,7 +64,7 @@
  7 5.692 [2.846, 8.538]
  8 8.538 [4.269, 12.807]
  9 12.807 [6.403, 19.210]
- 10 19.210 {@link BackOffPolicy#STOP}
+ 10 19.210 {@link BackOff#STOP}
  * </pre>
  *
  * <p>