doc/rules/index: keep rule types doc near the end

As this chapter is more meta than about rule keywords, keep it by the
end of the index, to have some semantic separation from the other
sections.

doc/userguide/rules/index.rst

@@ -55,6 +55,6 @@ Suricata Rules
    vlan-keywords
    ldap-keywords
    pgsql-keywords
-   rule-types
    email-keywords
+   rule-types
    rules-internals