Clarify multiple outputs in man page

Hi,

It took me way more than I thought to figure out the correct syntax of multiple outputs in the config file. The man page is a bit unclear, the correct syntax is:
```
image=eDP-1:~/.config/wallpapers/a.png
image=HDMI-A-2:~/.config/wallpapers/b.png
```
(repeating `image=` was not clear to me)

related to #373