DOC: lua: replace txn:close with txn:done in lua-api

It was missed in the last patch.
diff --git a/doc/lua-api/index.rst b/doc/lua-api/index.rst
index 5952398..f8cfee2 100644
--- a/doc/lua-api/index.rst
+++ b/doc/lua-api/index.rst
@@ -88,7 +88,7 @@
 
     function hello_world(txn)
        txn.res:send("hello world\n")
-       txn:close()
+       txn:done()
     end
 
 How to start HAProxy for testing this configuration: