Update mftest help

buildroot/bin/mftest

 
 usage() {
   echo "
-Usage: mftest [-t|--env=<env>] [-n|--num=<num>] [-m|--make] [-y|--build=<Y|n>]
+Usage: mftest [-t|--env=<env|index>] [-n|--num=<num>] [-m|--make] [-y|--build=<Y|n>]
        mftest [-a|--autobuild]
        mftest [-r|--rebuild]
        mftest [-u|--autoupload] [-n|--num=<num>]
 
 OPTIONS
-  -t --env         The environment of the test to apply / run. (As named in platformio.ini.)
+  -t --env         The environment to apply / run, or the menu index number.
   -n --num         The index of the test to run. (In file order.)
   -m --make        Use the make / Docker method for the build.
   -y --build       Skip 'Do you want to build this test?' and assume YES.