Fix array pack string slice

Adjust by two characters instead of one to account for endian ordering character as well

Fixes warning in Ruby 3.2 that will be an error in Ruby 3.3 <https://bugs.ruby-lang.org/issues/19150>

admin-interface/lib/rcon-client.rb

@@ -172,7 +172,7 @@ def build_packet(body, type=2, id=1, hide_content: false)
     packet_body = body
     packet_empty_string = ''
     packet = [packet_id, packet_type, packet_body, packet_empty_string] # Skip size for now
-    size = packet.pack(@@pack_string[1..]).length # Get packet size by packing what we have and checking length.
+    size = packet.pack(@@pack_string[2..]).length # Get packet size by packing what we have and checking length.
     packet.unshift(size)
     # log "Packet: " << packet.inspect
     packed = packet.pack(@@pack_string)