Suggested (minor) changes to get firefly to compile on arm64 linux

## Description

Some small changes are required to get firefly to successfully compile on arm64 linux. The solution is provided below for two files.

## Motivation

This will enable firefly to work on arm64 linux machines like Raspberry Pi4, Jetson Nano and others.

## Requirements

This fix will allow the wallet to compile without code modifications from the user or manual json commands.

## Are you planning to do it yourself in a pull request?

No, there is already an open branch for it at [ci-arm64](https://github.com/iotaledger/firefly/tree/feature/ci-arm64). However here are all the changes required:

File: packages/desktop/[package.json](https://github.com/iotaledger/firefly/blob/develop/packages/desktop/package.json)
+add one line below line 15:
"compile:linux-arm64": "electron-builder --linux --arm64",

File: packages/desktop/[README.md](https://github.com/iotaledger/firefly/blob/develop/packages/desktop/README.md)
+edit line 56 to become:
Change win to your operating system - mac, win, linux or linux-arm64.