Fixed docstring to render example code properly (#263)

Author: PhilippMisofCH

s2fft/utils/signal_generator.py

@@ -39,11 +39,11 @@ def complex_el_and_m_indices(L: int, min_el: int) -> tuple[np.ndarray, np.ndarra
 
     Equivalent to nested list-comprehension based implementation
 
-    ```
-    el_indices, m_indices = np.array(
-        [(el, m) for el in range(min_el, L) for m in range(1, el + 1)]
-    ).T
-    ```
+    .. code-block:: python
+
+        el_indices, m_indices = np.array(
+            [(el, m) for el in range(min_el, L) for m in range(1, el + 1)]
+        ).T
 
     For `L, min_el = 1024, 0`, this implementation is around 80x quicker in
     benchmarks compared to list-comprehension implementation.