docs: Specify nographic for running zephyr in Qemu

Zephyr-philosophers doesn't use video output but serial console.

Fixes: #docs#109



Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>

docs/build-flavours/zephyr-flavour.rst

@@ -38,7 +38,7 @@ You can test the image built for the qemu-x86 target by issuing:
 
 .. code-block:: console
 
-    $ runqemu qemu-x86
+    $ runqemu qemu-x86 nographic
 
 After successful bootup, the output of the application will be similar to: