fix: add missing deprecation leftover (#537)

Co-authored-by: Joan Touzet <wohali@users.noreply.github.com>

Author: janl

src/whatsnew/3.0.rst

@@ -153,6 +153,8 @@ The following features are deprecated in CouchDB 3.0 and will be removed in Couc
 
 * List functions (``/{db}/{ddoc}/_list``)
 
+* Update functions (``/{db}/{ddoc}/_update``)
+
 * Virtual hosts and ini-file rewrites
 
 * Rewrite functions (``/{db}/{ddoc}/_rewrite``)