General build instructions: give main example as shell command not Makefile

Signed-off-by: Thierry Laurion <insurgo@riseup.net>

Author: tlaurion

Installing-and-Configuring/Building-Heads/general.md

@@ -58,7 +58,7 @@ Generally, everything that is needed to flash the SPI flash of a board is a
 
  Make Heads for another board (`XXX` should be the name of your board in ./boards and YY the number of CPUs you want to build with):
 
- ```Makefile
+ ```shell
  docker run -e DISPLAY=$DISPLAY --network host --rm -ti -v $(pwd):$(pwd) -w $(pwd) tlaurion/heads-dev-env:latest -- make BOARD=XXX CPUS=YY
  ```