DOCSP-12245: fix regex setting name (#48)

* DOCSP-12245: fix regexp post processor setting name

Author: Chris Cho

source/kafka-sink-postprocessors.txt

@@ -599,7 +599,7 @@ Example
 
 .. code-block:: properties
 
-   field.renamer.mapping=[{"regexp":"^key\\.date.*$","pattern":"_","replace":"-"},{"regexp":"^value\\.crepes\\..*","pattern":"purchased","replace":"quantity"}]
+   field.renamer.regexp=[{"regexp":"^key\\.date.*$","pattern":"_","replace":"-"},{"regexp":"^value\\.crepes\\..*","pattern":"purchased","replace":"quantity"}]
 
 When the post processor is applied to the :ref:`sample key document <sample-key-document>`
 and the :ref:`sample value document <sample-value-document>`, it produces the