chat.tmux.conf: Fix base-index setting

"base-index" expects integer values, tmux prints a warning at load-time
but otherwise ignores the configuration line.

scripts/chat.tmux.conf

@@ -21,7 +21,7 @@ set-option -g	-- allow-rename	off
 set-option -g	-- set-titles	off
 set-option -g	-- renumber-windows	on
 # for the F1 binding, make hotkeys match window numbers
-set-option -g	-- base-index	on
+set-option -g	-- base-index	1
 
 
 # clients exit on network errors, restart them automatically