[json rpc] spec: update currency reference
[json rpc] spec: update currency reference

Closes: #3632
Approved by: thefallentree

aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra