[tui] Update fast mode plan usage copy (#13515)

## Summary
- update the /fast slash command description from 3X to 2X plan usage

## Testing
- not run (copy-only change)

codex-rs/tui/src/slash_command.rs

@@ -90,7 +90,7 @@ impl SlashCommand {
             SlashCommand::MemoryDrop => "DO NOT USE",
             SlashCommand::MemoryUpdate => "DO NOT USE",
             SlashCommand::Model => "choose what model and reasoning effort to use",
-            SlashCommand::Fast => "toggle Fast mode to enable fastest inference at 3X plan usage",
+            SlashCommand::Fast => "toggle Fast mode to enable fastest inference at 2X plan usage",
             SlashCommand::Personality => "choose a communication style for Codex",
             SlashCommand::Realtime => "toggle realtime voice mode (experimental)",
             SlashCommand::Settings => "configure realtime microphone/speaker",