Change the `user` to the one we have created for greetd.

diff --git a/config.toml b/config.toml
index c323af2..a90685a 100644
--- a/config.toml
+++ b/config.toml
@@ -13,4 +13,4 @@ command = "agreety --cmd /bin/sh"
 # The user to run the command as. The privileges this user must have depends
 # on the greeter. A graphical greeter may for example require the user to be
 # in the `video` group.
-user = "greeter"
+user = "greetd"