toolchain/crosstool-ng: be more explicit in the config file help

Some users seem to interpret:
    make ctng-menuconfig

as being a value that can be fit for the ct-ng config file.

Clarify that it is a command to run, not a possible value.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>