add this rex rst into index

Signed-off-by: Jialiang Liang <jiallian@amazon.com>

Author: RyanL1997

docs/user/ppl/index.rst

@@ -94,6 +94,8 @@ The query start with search command and then flowing a set of command delimited
 
   - `rename command <cmd/rename.rst>`_
 
+  - `rex command <cmd/rex.rst>`_
+
   - `search command <cmd/search.rst>`_
 
   - `show datasources command <cmd/showdatasources.rst>`_