use ${project.build.directory}

git-svn-id: https://svn.apache.org/repos/asf/maven/doxia/doxia/trunk@1125264 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/pom.xml b/pom.xml
index 74051ac..d10638c 100644
--- a/pom.xml
+++ b/pom.xml
@@ -255,7 +255,7 @@
         <directory>src/main/resources</directory>
       </resource>
       <resource>
-        <directory>${build.directory}/generated-site/xsd</directory>
+        <directory>${project.build.directory}/generated-site/xsd</directory>
         <includes>
           <include>**/*.xsd</include>
         </includes>