Correct HackMD license

There was a recent license change:

https://github.com/hackmdio/hackmd/pull/578
https://hackmd.io/s/r1_4j9_hZ

README.md

@@ -923,7 +923,7 @@ See https://github.com/n1trux/awesome-sysadmin#monitoring and https://github.com
 - [Boostnote](https://boostnote.io/) - The note-taking app for programmers that focuses on markdown, snippets, and customizability. ([Source Code](https://github.com/BoostIO/Boostnote/)) `GPL-3.0` `JavaScript`
 - [dillinger](http://dillinger.io/) - The last Markdown editor, ever. ([Source Code](https://github.com/joemccann/dillinger/)) `MIT` `Nodejs`
 - [draw.io](https://draw.io) - Diagram software for making flowcharts, process diagrams, org charts, UML, ER and network diagrams. ([Source Code](https://github.com/jgraph/drawio)) `Apache-2.0` `JavaScript`
-- [HackMD](https://www.hackmd.io) - Realtime collaborative markdown notes on all platforms. ([Source Code](https://github.com/hackmdio/hackmd/)) `MIT` `Node.js`
+- [HackMD](https://www.hackmd.io) - Realtime collaborative markdown notes on all platforms. ([Source Code](https://github.com/hackmdio/hackmd/)) `AGPL` `Node.js`
 - [Laverna](https://laverna.cc/) - Laverna is a Javascript note taking application with Markdown editor and encryption support. Consider it like open source alternative to Evernote. ([Demo](https://laverna.cc/index.html#notes), [Source Code](https://github.com/Laverna/laverna)) `MPL-2.0` `Nodejs`
 - [Leanote](http://leanote.org/) - Leanote, Not Just A Notepad! Open source cloud notepad. ([Demo](https://leanote.com/note), [Source Code](https://github.com/leanote/leanote)) `GPL-2.0` `Go`
 - [Markdown Edit](http://georgeosddev.github.com/markdown-edit/) - Online markdown editor/viewer. ([Source Code](https://github.com/georgeOsdDev/markdown-edit)) `MIT` `HTML5`