add protomoney Sub fix

[sameaste92]

What change does this introduce?

fix subtraction errors in Sub func in protomoney pkg

Why make this change?

currently the below example will panic

FortyDollarsUSD := &money.Money{
	CurrencyCode: "USD",
	Units:        40,
	Nanos:        0,
}
FiftyCentsUSD := &money.Money{
	CurrencyCode: "USD",
	Units:        0,
	Nanos:        500_000_000,
}

value := protomoney.Sub(FortyDollarsUSD, FiftyCentsUSD)

Is there anything you are unsure about?

Originally I had converted a and b to Amounts within the Sub() func.

But was unsure if we were avoiding that as no other math funcs use the main dosh pkg 🤷‍♂️

What issues does this relate to?

Maths is broken