message: [json rpc] spec: update currency reference
  Closes:
  Approved by: thefallentree
diff: @@ -573,9 +573,9 @@ A Libra account.
  <tr>
  <td>balance
  </td>
  - <td>integer
  + <td><a href="#amount---type">Amount</a>
  </td>
  - <td>Balance of given account in microlibra
  + <td>Balance of given account
  </td>
  </tr>
  <tr>
  @@ -597,6 +597,43 @@ A Libra account.
  </table>
  +##
  +
  +---
  +
  +
  +
  +## Amount - type
  +
  +### Attributes
  +
  +<table>
  + <tr>
  + <td><strong>Name</strong>
  + </td>
  + <td><strong>Type</strong>
  + </td>
  + <td><strong>Description</strong>
  + </td>
  + </tr>
  + <tr>
  + <td>amount
  + </td>
  + <td>u64
  + </td>
  + <td>
  + amount in currency microunits
  + </td>
  + </tr>
  + <tr>
  + <td>currency
  + </td>
  + <td>string
  + </td>
  + <td>currency string identifier
  + </td>
  + </tr>
  +</table>
  ##
  @@ -1169,7 +1206,7 @@ Event emitted when an account received a payment.
  <tr>
  <td><strong>amount</strong>
  </td>
  - <td>u64
  + <td><a href="#amount---type">Amount</a>
  </td>
  <td>Amount received from the sender of the transaction
  </td>
  @@ -1231,7 +1268,7 @@ Event emitted when an account sends a payment.
  <tr>
  <td><strong>amount</strong>
  </td>
  - <td>integer
  + <td><a href="#amount---type">Amount</a>
  </td>
  <td>Amount transferred in a transaction
  </td>
  