Change Salt50 promo code to Salt20

[nmelehan]

Change was performed by executing this command on an up-to-date develop branch (using the macOS version of sed):

grep Salt50 -Rl docs | xargs sed -i '' -e 's/Salt50/Salt20/' -e "s/promo_code_amount: '50'/promo_code_amount: '20'/"

[nmelehan]

This will be directly added to #2238, so closing this PR