Fix broken link to application icon in README.md

Fixes error:
```
Refused to load the image 'https://github.com/downloads/jberkel/sms-backup-plus/smsbackup72.png' because it violates the following Content Security Policy directive: "img-src 'self' data: assets-cdn.github.com identicons.github.com www.google-analytics.com collector.githubapp.com *.githubusercontent.com *.gravatar.com *.wp.com".
```