fix(droppy): Change demo link

Change demo link to https in order to fix the following error:
[L0744]  http://droppy.silverwind.io/ end of file reached

Signed-off-by: Rodrigo Avelino <rodrigo@avelino.org>

README.md

@@ -741,7 +741,7 @@ See https://github.com/n1trux/awesome-sysadmin#distributed-filesystems
 
 - [BoZoN](https://github.com/broncowdd/BoZoN) - Minimalist Drag and drop file sharing app. `AGPL-3.0` `PHP`
 - [Coquelicot](https://coquelicot.potager.org/) - Coquelicot is a “one-click” file sharing web application with a focus on protecting users’ privacy. ([Source Code](https://coquelicot.potager.org/gitweb/?p=coquelicot.git)) `AGPL-3.0` `Ruby`
-- [droppy](https://github.com/silverwind/droppy) - droppy is a self-hosted cloud server with an interface similar to desktop file managers and has capabilities to edit files on-the-fly as well as view and playback media directly in the browser. ([Demo](http://droppy.silverwind.io/)) `BSD-2-Clause` `Nodejs`
+- [droppy](https://github.com/silverwind/droppy) - droppy is a self-hosted cloud server with an interface similar to desktop file managers and has capabilities to edit files on-the-fly as well as view and playback media directly in the browser. ([Demo](https://droppy.silverwind.io/)) `BSD-2-Clause` `Nodejs`
 - [elixire](https://elixi.re) - Simple yet advanced screenshot uploading and link shortening service. ([Source Code](https://gitlab.com/elixire/elixire), [Clients](https://gitlab.com/elixire/elixiremanager)) `AGPL-3.0` `Python`
 - [fibridge](https://github.com/anderspitman/fibridge-proxy-rs) - Stream huge files out of your browser without having to upload. ([Demo](https://fbrg.xyz/)) `MIT` `Rust`
 - [FileShelter](https://github.com/epoupon/fileshelter) - FileShelter is a self-hosted software that allows you to easily share files over the Internet. ([Demo](http://fileshelter.demo.poupon.io/)) `GPL-3.0` `C++`