chore(AIP-143): fix money.proto directory reference (#1455)

Author: MEnnabah

aip/general/0143.md

@@ -70,7 +70,7 @@ Fields representing currency **must** use [ISO-4217 currency codes][iso-4217],
 such as `USD` or `CHF`, and the field **must** be called `currency_code`.
 
 **Note:** For representing an amount of money in a particular currency, rather
-than the currency code itself, use [`google.protobuf.Money`][money].
+than the currency code itself, use [`google.type.Money`][money].
 
 ### Language