Fix check_pico_sdk error message

[hayaokimura]

Error message of pico-extras version is not truth.

before

rake aborted!
pico-sdk 1.5.1 is not checked out!

Tips for dealing with:

cd $PICO_SDK_PATH && git pull && git checkout 1.5.1 && git submodule update --recursive

after

rake aborted!
pico-extras sdk-1.5.1 is not checked out!

Tips for dealing with:

cd $PICO_EXTRAS_PATH && git pull && git checkout sdk-1.5.1 && git submodule update --recursive