DOC: lua: fix core.register_action typo

"converter" was used in place of "action" as a result of a copy-paste
error probably.

Also, rephrasing the "actions" keyword explanation to prevent confusion
between action name (which is the name of new action about to be created)
and action facilities where we want to expose the new action.

This could be backported to every stable versions.

(cherry picked from commit 7e9aea789f4412bbcb149f9954b08da861c2fc7e)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/doc/lua-api/index.rst b/doc/lua-api/index.rst
index d2720d2..96d48ae 100644
--- a/doc/lua-api/index.rst
+++ b/doc/lua-api/index.rst
@@ -480,11 +480,11 @@
   used in HAProxy with the prefix "lua.". An action gets a TXN object class as
   input.
 
-  :param string name: is the name of the converter.
-  :param table actions: is a table of string describing the HAProxy actions who
-   want to register to. The expected actions are 'tcp-req', 'tcp-res', 'http-req'
-   or 'http-res'.
-  :param function func: is the Lua function called to work as converter.
+  :param string name: is the name of the action.
+  :param table actions: is a table of string describing the HAProxy actions
+   facilities where to expose the new action. Expected facilities  are:
+   'tcp-req', 'tcp-res', 'http-req' or 'http-res'.
+  :param function func: is the Lua function called to work as an action.
   :param integer nb_args: is the expected number of argument for the action.
    By default the value is 0.