docs: update PubSweet logo in README

README.md

 <div width="100%" align="center">
-  <img src="https://gitlab.coko.foundation/pubsweet/pubsweet/raw/master/assets/rgb-medium.jpg" width="300" />
-  <h2>The open toolkit for building publishing workflows</h2>
+  <img src="https://gitlab.coko.foundation/pubsweet/pubsweet/raw/master/assets/logo-pubsweet.png" width="300" />
+  <h2>PubSweet: The open toolkit for building publishing workflows</h2>
 </div>
 
 | [![MIT license](https://img.shields.io/badge/license-MIT-e51879.svg)](https://gitlab.coko.foundation/pubsweet/pubsweet/raw/master/LICENSE) [![mattermost](https://img.shields.io/badge/mattermost_chat-coko%2Fpubsweet-blue.svg)](https://mattermost.coko.foundation/coko/channels/pubsweet) [![Commitizen friendly](https://img.shields.io/badge/commitizen-friendly-brightgreen.svg)](http://commitizen.github.io/cz-cli/) |