rpc: fix name of json nep5balance field

Problem: wrong json field name of result.NEP5Balances.Balances
(see for details:
https://github.com/neo-project/neo-modules/blob/master-2.x/RpcNep5Tracker/RpcNep5Tracker.cs#L295)

Solution: rename field from `balances` to `balance` to be consistent with C# client

pkg/rpc/response/result/nep5.go

@@ -8,7 +8,7 @@ import (
 
 // NEP5Balances is a result for the getnep5balances RPC call.
 type NEP5Balances struct {
-	Balances []NEP5Balance `json:"balances"`
+	Balances []NEP5Balance `json:"balance"`
 	Address  string        `json:"address"`
 }