Fix allowed method for ddoc update function

The semantics of PUT [1] are arguably incorrect for executing an
update function on a specified document, since nothing is being
created or updated. Also it's not clear why it would be different from
the POST used for executing an update function for `null` document. [2]

Additionally, the example code-block in this section uses POST:

        POST /recipes/_design/recipe/_update/ingredients/SpaghettiWithMeatballs HTTP/1.1

All which lead me to believe that the PUT is a typo.

The actual behavior of CouchDB doesn't help here, since it currently
accepts any method whatsoever:

$ curl -v -u adm localhost:15984/db-1/_design/ddoc/_update/change -X BOGUS

> BOGUS /db-1/_design/bar/_update/change HTTP/1.1
>
< HTTP/1.1 200 OK
< content-length: 11
< content-type: text/html; charset=utf-8
<
* Connection #0 to host localhost left intact
Empty World* Closing connection 0

[1] https://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html
[2] https://docs.couchdb.org/en/latest/api/ddoc/render.html#post--db-_design-ddoc-_update-func
diff --git a/src/api/ddoc/render.rst b/src/api/ddoc/render.rst
index d7c7b0e..8324bcd 100644
--- a/src/api/ddoc/render.rst
+++ b/src/api/ddoc/render.rst
@@ -352,7 +352,7 @@
 ``/db/_design/design-doc/_update/update-name/doc-id``
 =====================================================
 
-.. http:put:: /{db}/_design/{ddoc}/_update/{func}/{docid}
+.. http:post:: /{db}/_design/{ddoc}/_update/{func}/{docid}
     :synopsis: Executes an update function against the specified document
 
     Executes :ref:`update function <updatefun>` on server side for the specified