doc/guides: Improve code block

Author: LasseRosenow

doc/guides/build-system/build_system.mdx

@@ -29,7 +29,7 @@ variables from the command line. For example, you can easily specify the target
 platform, using the sample Makefile, by invoking make like this:
 
 ```shell
-make BOARD=iotlab-m3
+BOARD=iotlab-m3 make
 ```
 
 ## Makefile targets