Broken link to blog post in README.md

## Description

In README.md there's a broken link:
```
To read more about the motivation behind the _Demikernel_, check out this
[blog post](http://irenezhang.net/blog/2019/05/21/demikernel.html).
```

The link is actually: https://irenezhang.net/2019/05/21/demikernel/