fix warning in new platformio (SlimeVR#176)

Author: unlogisch04

platformio.ini

@@ -15,7 +15,7 @@
 lib_deps=
   https://github.com/SlimeVR/CmdParser.git
 monitor_speed = 115200
-monitor_flags = --echo
+monitor_echo = yes
 monitor_filters = colorize
 ;monitor_rts = 0
 ;monitor_dtr = 0