Fix readme commands

Author: Tim Hutt

os-boot/README.md

@@ -7,15 +7,15 @@ The Sail model implements a very simple platform based on the one implemented by
 ## Build your own ELF
 
 ```bash
-make -j4 -C linux
+make -C linux -j4
 ```
 
 This will generate `os-boot/linux/build/fw_payload.elf`.
 
 ### Boot ELF
 
 ```bash
-make sail
+make -C linux sail
 ```
 
 You should see the OpenSBI banner after a few seconds. Eventually it will crash when it fails to find an `init` process which the image does not currently include.