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

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

Author: junyixu

docs/src/dev/how_it_works.md

@@ -86,7 +86,7 @@ the result. For example, to calculate the partial derivatives for the gradient o
                          \vdots \\
                          x_N + \epsilon_N
                      \end{bmatrix}
-\to
+\mapsto
 f(\vec{x}_{\epsilon}) = f(\vec{x}) + \sum_{i=1}^N \frac{\delta f(\vec{x})}{\delta x_i} \epsilon_i
 ```
 
@@ -111,7 +111,7 @@ two calls to ``f`` to evaluate the gradient:
                         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:
@@ -122,7 +122,7 @@ f(\vec{x}_{\epsilon}) = f(\vec{x}) + \frac{\delta f(\vec{x})}{\delta x_1} \epsil
                         x_3 + \epsilon_1 \\
                         x_4 + \epsilon_2
                      \end{bmatrix}
-\to
+\mapsto
 f(\vec{x}_{\epsilon}) = f(\vec{x}) + \frac{\delta f(\vec{x})}{\delta x_3} \epsilon_1 + \frac{\delta f(\vec{x})}{\delta x_4} \epsilon_2.
 ```