docs: stop telling sc users to press a key only the TUI binds

The sc chooser runs a plain `tmux attach-session` and binds nothing, so
F1 does not detach from it; only `codeman tui`'s attach claims that key,
and only for its own duration. The line it replaced was wrong too (the
socket's prefix is C-b, not C-a), so name the real chord and say which
command gives you the single key instead.

README.md

@@ -684,7 +684,7 @@ sc 2            # Quick attach to session 2
 sc -l           # List sessions
 ```
 
-Single-digit selection (1-9), color-coded status, token counts, auto-refresh. Come back from an attached pane with `F1`.
+Single-digit selection (1-9), color-coded status, token counts, auto-refresh. `sc` hands you to a plain `tmux attach`, so you leave the pane with tmux's own detach chord: `Ctrl+B`, release, then `d`. (`codeman tui` is the one that gives you a single `F1` instead, because its attach claims that key for the duration.)
 
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