fix: Replace \to with \mapsto due to the nonstandard meaning of \to

Co-authored-by: Michael Abbott <[email protected]>

Author: junyixu

Diff for: docs/src/dev/how_it_works.md

@@ -105,13 +105,13 @@ two calls to ``f`` to evaluate the gradient:
 ```
 1st call:
 ```math
-\to \vec{x}_{\epsilon} = \begin{bmatrix}
+\vec{x}_{\epsilon} = \begin{bmatrix}
                         x_1 + \epsilon_1 \\
                         x_2 + \epsilon_2 \\
                         x_3 \\
                         x_4
                      \end{bmatrix}
-\to
+\mapsto
 f(\vec{x}_{\epsilon}) = f(\vec{x}) + \frac{\delta f(\vec{x})}{\delta x_1} \epsilon_1 + \frac{\delta f(\vec{x})}{\delta x_2} \epsilon_2,
 ```
 2nd call: