[*] MO: Referral program: display currency in account page next to the voucher value

In the referral program account page the text was missing the currency sign. This commit fix it:
Before:
`...he or she will receive a 5 voucher and you will receive your own voucher worth 5`
After:
`...he or she will receive a 5 $ voucher and you will receive your own voucher worth 5 $.`