Documentation update: Unit Identifiers

The Amatino API HTTP documentation has been updated. This update corrects errors in references to Global Units and Custom Units. Previously, the documentation stated that such units should be identified by their string codes. For example, 'usd' for U.S. Dollars.

Now, the documentation states that units should be identified by their integer identifiers. For example, 5 for U.S. Dollars.

This error was the result of some earlier experiments with removing integer unit identifiers in favour of string identifiers. The experiments did not go well, and lots of problems resulted. So, I reverted to integer identifiers, but forgot to update the documentation.