Fixed coefficients in qubit kata explanation. (#2225)

Indeed, atan2(y, x) is present in many languages. We also have
ArcTan2(y, x) in Q#, and it takes y as the first argument as expected.
The text describes an example of a phase of a complex number $\frac{1 -
i}{2}$. Here x=1/2 and y=-1/2. Previous text had these coefficients
switched. Now it is fixed.
Also changed wording from "the phase of |0>" to "the phase of |0>
amplitude", which seems to be a better wording as the paragraph talks
about phases of complex numbers.

Co-authored-by: Dmitry Vasilevsky <[email protected]>

Author: DmitryVasilevsky

katas/content/qubit/index.md

@@ -142,7 +142,7 @@ Complex numbers have a parameter called the phase. If a complex number $z = x +
 > between $-\pi$ and $\pi$ that has $\cos \theta = x$ and $\sin \theta = y$. Unlike using $\tan^{-1}(\frac{y}{x})$, `atan2` computes
 > the correct quadrant for the angle, since it preserves information about the signs of both sine and cosine of the angle.
 
-The probability amplitudes $\alpha$ and $\beta$ are complex numbers, therefore $\alpha$ and $\beta$ have a phase. For example, consider a qubit in state $\frac{1 + i}{2}\ket{0} + \frac{1 - i}{2}\ket{1}$. If you do the math, you see that the phase of $\ket{0}$ is $atan2(\frac12, \frac12) = \frac{\pi}{4}$, and the phase of $\ket{1}$ is $atan2(\frac12, -\frac12) = -\frac{\pi}{4}$. The difference between these two phases is known as **relative phase**.
+The probability amplitudes $\alpha$ and $\beta$ are complex numbers, therefore $\alpha$ and $\beta$ have a phase. For example, consider a qubit in state $\frac{1 + i}{2}\ket{0} + \frac{1 - i}{2}\ket{1}$. If you do the math, you see that the phase of $\ket{0}$ amplitude is $atan2(\frac12, \frac12) = \frac{\pi}{4}$, and the phase of $\ket{1}$ amplitude is $atan2(-\frac12, \frac12) = -\frac{\pi}{4}$. The difference between these two phases is known as **relative phase**.
 
 Multiplying the state of the entire system by $e^{i\theta}$ doesn't affect the relative phase: $\alpha\ket{0} + \beta\ket{1}$ has the same relative phase as $e^{i\theta}\big(\alpha\ket{0} + \beta\ket{1}\big)$. In the second expression, $\theta$ is known as the system's **global phase**.