change to fix the JavaDoc error during build

Review at http://gwt-code-reviews.appspot.com/1478801

Review by: pdr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10444 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/doc/build.xml b/doc/build.xml
index a66d058..5be175f 100644
--- a/doc/build.xml
+++ b/doc/build.xml
@@ -94,6 +94,7 @@
       <arg value="Google Web Toolkit API Reference" />
       <arg value="-header" />
       <arg value="GWT ${gwt.version}" />
+      <arg value="-package" />
       <arg value="-encoding"/>
       <arg value="UTF-8"/>
       <arg value="-d" />