Fix wrong index preparation in mrdb:index_read_/3

In the case when index is of type {_}, index is returned as is
instead of putting it into a tuple {Ix, ordered}.

An example

Given the following index plugin implementation:
-module(key6).

-record(cgh, {key, val}).

-export([k6/3
        , add_index/0
        , create_table/0
        , tst_bad_type/0
    ]).

k6(_, _, #cgh{key = {Name, K8, _Cid, _Vn}}) ->
    [list_to_binary([atom_to_binary(Name),
     list_to_binary(lists:sublist(K8, 6))])].

add_index() ->
    mnesia_schema:add_index_plugin({k6}, key6, k6).

create_table() ->
    mnesia:create_table(cgh,
    [{attributes, record_info(fields, cgh)},
     {type, ordered_set},
     {index, [{k6}]},
     {rocksdb_copies, [node()]},
     {local_content, true}]).

tst_bad_type() ->
    E1 = #cgh{key = {n1, "u1vgrjkh",{ecgi,238,6,213020,50},3}, val = ok},
    mnesia:dirty_write(E1),
    Index1 = list_to_binary([atom_to_binary(n1),
                             list_to_binary(lists:sublist("u1vgrjkh", 6))]),
    mrdb:index_read(cgh, Index1, {k6}).

-- testing --
Eshell V14.2.2 (press Ctrl+G to abort, type help(). for help)
(rock@bang)2> mnesia:create_schema([node()]).
ok
(rock@bang)3> mnesia:start().
ok
(rock@bang)4> mnesia_rocksdb:register().
{ok,rocksdb_copies}
(rock@bang)5> l(key6).
{module,key6}
(rock@bang)6> key6:add_index().
{atomic,ok}
(rock@bang)7> key6:create_table().
{atomic,ok}
(rock@bang)8> key6:tst_bad_type().
** exception exit: {aborted,{bad_type,{cgh,index,{k6}}}}
     in function  mnesia:abort/1 (mnesia.erl, line 362)
     in call from mrdb:ensure_ref/1 (~/projects/erlang/github/mnesia_rocksdb/src/mrdb.erl, line 609)
     in call from mrdb:index_read_/3 (~/projects/erlang/github/mnesia_rocksdb/src/mrdb.erl, line 945)

-- after patch --

(rock@bang)9> c(mrdb).
Recompiling ~/projects/erlang/github/mnesia_rocksdb/src/mrdb.erl
{ok,mrdb}
(rock@bang)10> key6:tst_bad_type().
[{cgh,{n1,"u1vgrjkh",{ecgi,238,6,213020,50},3},ok}]
(rock@bang)11>

src/mrdb.erl

@@ -938,7 +938,7 @@ index_read_(#{name := Main, semantics := Sem} = Ref, Val, Ix) ->
             _ when is_atom(Ix) ->
                 {attr_pos(Ix, Ref), ordered};
             {_} ->
-                Ix;
+                {Ix, ordered};
             _ when is_integer(Ix) ->
                 {Ix, ordered}
         end,