NIFI-4692 Fix AttributesToJSON property docs to match functionality

Signed-off-by: Pierre Villard <pierre.villard.fr@gmail.com>

This closes #5313.
diff --git a/nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/AttributesToJSON.java b/nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/AttributesToJSON.java
index 46340bd..eb1b52a 100644
--- a/nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/AttributesToJSON.java
+++ b/nifi-nar-bundles/nifi-standard-bundle/nifi-standard-processors/src/main/java/org/apache/nifi/processors/standard/AttributesToJSON.java
@@ -115,7 +115,7 @@
 
     public static final PropertyDescriptor NULL_VALUE_FOR_EMPTY_STRING = new PropertyDescriptor.Builder()
             .name(("Null Value"))
-            .description("If true a non existing or empty attribute will be NULL in the resulting JSON. If false an empty " +
+            .description("If true a non existing selected attribute will be NULL in the resulting JSON. If false an empty " +
                     "string will be placed in the JSON")
             .required(true)
             .allowableValues("true", "false")