stbt_camera_calibrate.fit_fn: Fix doctest failure
stbt_camera_calibrate.fit_fn: Fix doctest failure

The failure was:

    Expected:
        0.0 56.0
    Got:
        0.0 56.00000000000013

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester