chore: fast root docs equation (#331)

Tiny minor fast_root.cairo doc update.
Equation seems off for a root function. Matches a power function.

<img width="453" alt="image"
src="https://github.com/user-attachments/assets/71b15e6b-c40f-4ea1-b320-2fd5e4eacd71">

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- [ ] Bugfix
- [ ] Feature
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packages/math/src/fast_root.cairo

@@ -1,7 +1,7 @@
 //! # Fast root algorithm using the Newton-Raphson method
 use super::pow;
 
-/// Newton-Raphson optimization to solve the equation x^r = a.
+/// Newton-Raphson optimization to solve the equation a^r = x.
 /// The optimization has a quadratic convergence rate.
 /// # Arguments
 /// * ` x ` - The number to calculate the root of