Multiply the complex numbers.
\[
z_1z_2 = (1 - i) \cdot i = i - i^2 = i + 1
\]
Step 2: Determine the argument.
The complex number \( i + 1 \) is in the first quadrant of the complex plane. The angle corresponding to a vector with equal real and imaginary parts is \(\frac{\pi}{4}\).