Fix tab completion output with OTP 26

In OTP 26 requires explicitly defining `expand_below: false`
when starting the group and defaults to expanding below the prompt line.

[`ssh_cli.erl` now defaults to `expand_below: false`](erlang/otp@81562f7#diff-d1641a7f3fc9ba8d592c295bc91b373637dd6660ea95a8441f501df3792642f7) and so this change matches that setting since expanding below is currently
not supported.

This is the current behavior in previous OTP versions

lib/extty.ex

@@ -97,7 +97,8 @@ defmodule ExTTY do
   defp start_shell(state) do
     %{
       state
-      | group: :group.start(self(), shell_spawner(state), [{:echo, true}]),
+      | group:
+          :group.start(self(), shell_spawner(state), [{:echo, true}, {:expand_below, false}]),
         buf: @empty_buf
     }
   end