Fixes license badge on README.
<td colspan="2">
<a href="https://rusoto.github.io/rusoto/" title="API Docs"><img src="https://img.shields.io/badge/API-docs-blue.svg" alt="api-docs-badge"></img></a>
<a href="https://crates.io/crates/rusoto_core" title="Crates.io"><img src="https://img.shields.io/crates/v/rusoto_core.svg" alt="crates-io"></img></a>
- <a href="#license" title="License: MIT"><img src="https://img.shields.io/crates/l/rusoto_core.svg" alt="license-badge"></img></a>
+ <a href="#license" title="License: MIT"><img src="https://img.shields.io/badge/license-MIT-blue.svg" alt="license-badge"></img></a>
</td>
</tr>
</table>
