Readme: show "npm downloads" count per month (not per week)

The weekly numbers fluctuate a lot - let's see if we get more accurate
and stable results with a monthly count.

README.md

@@ -5,7 +5,7 @@
 
 [![npm@latest](https://img.shields.io/npm/v/kaitai-struct-compiler/latest)](https://www.npmjs.com/package/kaitai-struct-compiler/v/latest)
 [![npm@next](https://img.shields.io/npm/v/kaitai-struct-compiler/next)](https://www.npmjs.com/package/kaitai-struct-compiler/v/next)
-[![npm downloads](https://img.shields.io/npm/dw/kaitai-struct-compiler?label=npm%20downloads)](https://www.npmtrends.com/kaitai-struct-compiler)
+[![npm downloads](https://img.shields.io/npm/dm/kaitai-struct-compiler?label=npm%20downloads)](https://www.npmtrends.com/kaitai-struct-compiler)
 
 This project is an official reference compiler for [Kaitai Struct](http://kaitai.io) project.