Slide in video at 2:58 is different from what is explained.

[bikash119]

Please check the attached screenshot. This issue is also exist in slide 13 of unit 6.4.
The derivative of $\sigma(z)$ at 0.5 should be 0.5(1-0.5) = 0.25, but the slide shows it as 0.5(0.5-0).