docs: use a visable arraw character for web page

Author: wfxr

README.md

@@ -71,8 +71,8 @@ set -g @tmux_power_user_icon 'U'
 set -g @tmux_power_session_icon 'S'
 set -g @tmux_power_upload_speed_icon '↑'
 set -g @tmux_power_download_speed_icon '↓'
-set -g @tmux_power_left_arrow_icon ''
-set -g @tmux_power_right_arrow_icon ''
+set -g @tmux_power_left_arrow_icon '<'
+set -g @tmux_power_right_arrow_icon '>'
 ```
 *The default icons use glyphs from [nerd-fonts](https://github.com/ryanoasis/nerd-fonts).*