fix theme image

Author: jjw24

how-to-create-a-theme.md

@@ -12,7 +12,7 @@ The theme file allows you to set the following parts. Each style has a key, and
 
 (*There is a possibility of changing/deleting this part depending on the version.*)
 
-<img src="/assets/themelayout.png" style="zoom:50%;" />
+<img src="https://github.com/Flow-Launcher/docs/blob/main/assets/themelayout.png" style="zoom:50%;" />
 
 ##### WindowBorderStyle