tmux: Fix display bug in session chooser

Session names would be cut off by the preview window.
Now, the preview window is generally *close* to the session names but
not overlapping them, which looked weird.

tmux/.config/sh/alias.d/tmux.sh

@@ -24,7 +24,7 @@ _fzf_tmux_list_sessions() {
     --border \
     --prompt="Session> " \
     --preview="tmux_pane_tree {}" \
-    --preview-window=right:99% \
+    --preview-window=right:80% \
     --print-query
 }