Fix missing attribute

- Use spring-session-data-mongodb-dir
- Update example to use symlink location

spring-session-docs/modules/ROOT/examples/spring-session-data-mongodb-dir

@@ -0,0 +1 @@
+../../../../spring-session-data-mongodb/

spring-session-docs/modules/ROOT/pages/http-session.adoc

@@ -92,7 +92,7 @@ If you would like to provide custom Jackson modules you can do it by explicitly
 
 [source,java,indent=0]
 ----
-include::{code-dir}/src/test/java/org/springframework/session/data/mongo/integration/MongoRepositoryJacksonITest.java[tag=sample]
+include::{spring-session-data-mongodb-dir}src/integration-test/java/org/springframework/session/data/mongo/integration/MongoRepositoryJacksonITest.java[tag=sample]
 ----
 
 ===== JdkMongoSessionConverter

spring-session-docs/spring-session-docs.gradle

@@ -61,6 +61,7 @@ asciidoc:
     gh-samples-url: "$ghUrl/spring-session-samples/"
     samples-dir: "example${dollar}spring-session-samples/"
     session-jdbc-main-resources-dir: "example${dollar}session-jdbc-main-resources-dir/"
+    spring-session-data-mongodb-dir: "example${dollar}spring-session-data-mongodb-dir/"
     docs-test-dir: "example${dollar}java/"
     websocketdoc-test-dir: 'example${dollar}java/docs/websocket/'
     docs-test-resources-dir: "example${dollar}resources/"