Update README.md
Cause Github don't like <img src="...svg">
<div align="center">
<a href="https://stenci.la">
- <img src="https://raw.githubusercontent.com/stencila/stencila/master/images/logo-name-beta.svg" alt="Stencila">
+ <img src="https://raw.githubusercontent.com/stencila/stencila/master/images/logo-name.png" alt="Stencila">
</a>
<br>
<a href="#download">Download</a>&nbsp;|&nbsp;
