refactor ssh-agent cookbook example

[amtoine]

this PR hopefully makes the ssh-agent example in the cookbook easier to read 😇

i've also removed the note about ssh-add as it does not appear to be useful at all 🤔
i use this ssh-agent everyday without ever running ssh-add 😕

[sholderbach]

Seems to be good, (man are those transpose options confusing for my taste)

[amtoine]

man are those transpose options confusing for my taste

yeah i agree 😆
i always have to play with the combinations of -r, -i and -d to get what i want 👀