README: Fix image for showcase missionbit.org

Set the correct path, so that the image is shown in the README.

Author: FinnWoelm

README.md

@@ -165,7 +165,7 @@ For more on this, see: [Issue #7](https://github.com/FinnWoelm/next-on-netlify/i
 
 The following sites are built with `next-on-netlify`:
 
-![missionbit.org](https://raw.githubusercontent.com/FinnWoelm/next-on-netlify/assets/showcase-missionbit.png)  
+![missionbit.org](https://raw.githubusercontent.com/FinnWoelm/next-on-netlify/master/assets/showcase-missionbit.png)  
 [missionbit.org](https://www.missionbit.org/) ([#18](https://github.com/FinnWoelm/next-on-netlify/pull/18#issuecomment-643828966))
 
 Are building something awesome with `next-on-netlify`? 🔥 Let me know and we will feature it here :)