python3: correctly fix PYTHON3_PATH

Commit 6251ccf0 ('python3: fix the
value of PYTHON3_PATH') tried to fix the value of PYTHON3_PATH, but
did it incorrectly: it changed PYTHON_VERSION_MAJOR to
PYTHON_VERSION3_MAJOR, while it should have been
PYTHON3_VERSION_MAJOR.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>