fix(documentation): use bigint instead of integer

[davidalencar]

Updating the documentation to guide users on creating their complex type using bigint instead of integer.

[davidalencar]

When we first implemented it, we simply followed the documentation and created our complex type using integer, which corresponds to an int32. However, when financial transactions in our application started to involve larger amounts, we exceeded the capacity of int32. By that point, the complex type money_with_currency was already in use across multiple tables in PostgreSQL and couldn’t be modified. We were forced to create a new type in the database using bigint (int64) and a new type in Elixir, which we named bigmoney. We had to migrate the data from the old fields that used the complex type with integer, and this migration took weeks and a lot of effort.

This simple change I'm proposing will save a lot of work and headaches. =)