ProjectCreationValidatorIT: Fix expected substring

Before this change, the test was failing since the actual string change.

Change-Id: Ifc7e3cf486487a8628a18023bb536c540d85a7f9
diff --git a/src/test/java/com/ericsson/gerrit/plugins/projectgroupstructure/ProjectCreationValidatorIT.java b/src/test/java/com/ericsson/gerrit/plugins/projectgroupstructure/ProjectCreationValidatorIT.java
index 8006b23..8f53648 100644
--- a/src/test/java/com/ericsson/gerrit/plugins/projectgroupstructure/ProjectCreationValidatorIT.java
+++ b/src/test/java/com/ericsson/gerrit/plugins/projectgroupstructure/ProjectCreationValidatorIT.java
@@ -81,7 +81,7 @@
     // Creation is rejected when root project name contains slashes
     RestResponse r = userRestSession.put("/projects/" + Url.encode("a/parentProject"), in);
     r.assertConflict();
-    assertThat(r.getEntityContent()).contains("Root project name cannot contains slashes");
+    assertThat(r.getEntityContent()).contains("Root project names cannot contain slashes");
   }
 
   @Test