Modify number of types of chains (#1089)

Changed number of types of chains to make it consistent with the rest of
the docs

docs/modules/chains/how_to_guides.rst

@@ -4,7 +4,7 @@ How-To Guides
 A chain is made up of links, which can be either primitives or other chains.
 Primitives can be either `prompts <../prompts.html>`_, `llms <../llms.html>`_, `utils <../utils.html>`_, or other chains.
 The examples here are all end-to-end chains for specific applications.
-They are broken up into three categories:
+They are broken up into four categories:
 
 1. `Generic Chains <./generic_how_to.html>`_: Generic chains, that are meant to help build other chains rather than serve a particular purpose.
 2. `CombineDocuments Chains <./combine_docs_how_to.html>`_: Chains aimed at making it easy to work with documents (question answering, summarization, etc).