Add "akka" prefix to TCK code sample (#29529)

The current code sample for trying out the TCK misses the "akka" prefix
which means that the TCK test will fail with an error regarding not
specifying a journal plugin (because the journal plugin property is
wrong in the sample)

Fixes #29528

akka-docs/src/test/java/jdocs/persistence/LambdaPersistencePluginDocTest.java

@@ -139,7 +139,7 @@ public class LambdaPersistencePluginDocTest {
           public MyJournalSpecTest() {
             super(
                 ConfigFactory.parseString(
-                    "persistence.journal.plugin = "
+                    "akka.persistence.journal.plugin = "
                         + "\"akka.persistence.journal.leveldb-shared\""));
           }