Fix melpa badge (#11)

problem:
The current melpa badge is broken, it's url points to:
http://melpa.milkbox.net/#/lsp-sourcekit

http://melpa.milkbox.net/ shows: This site can’t be reached
https://www.milkbox.net/ shows: Squarespace - Website Expired

cause:
The switch from melpa.milkbox.net to melpa.org seems to have happened 5 years ago:
https://www.reddit.com/r/emacs/comments/2k2kmv/melpamilkboxnet_is_now_melpaorg/

The milkbox url must have expired within the last two years, since the current badge was added.

solution:
Used the melpa badge instead: https://melpa.org/#/lsp-sourcekit

Author: duianto

README.md

@@ -1,4 +1,4 @@
-[![Melpa Status](http://melpa.milkbox.net/packages/lsp-sourcekit-badge.svg)](http://melpa.milkbox.net/#/lsp-sourcekit)
+[![MELPA](https://melpa.org/packages/lsp-sourcekit-badge.svg)](https://melpa.org/#/lsp-sourcekit)
 
 # lsp-sourcekit