plot_strings.py: Replace np.complex with complex

plot_strings.py: Replace np.complex with complex

Replace np.complex with complex to fix numpy error:

AttributeError: module 'numpy' has no attribute 'complex'.
`np.complex` was a deprecated alias for the builtin `complex`. To avoid this error in existing code, use `complex` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.complex128` here.
The aliases was originally deprecated in NumPy 1.20; for more details and guidance see the original release note at:
    https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Collin Funk <collin.funk1@gmail.com>