doc: correct magnetic field orientation at the origin

Previously a point defect introduced by an incorrect treatment
of the origin produced erroneous scattering. We also update the text
to reflect the actual magnetic potential used.

Closes #156.

doc/source/code/figure/magnetic_texture.py.diff

@@ -40,7 +40,7 @@
      theta = (tanh(r_tilde) - 1) * (pi / 2)
  
      if r == 0:
-         m_i = [0, 0, 1]
+         m_i = [0, 0, -1]
      else:
          m_i = [
              (x / r) * sin(theta),

doc/source/tutorial/operators.rst

@@ -39,7 +39,7 @@ We shall take the following form for :math:`\mathbf{m}_i`:
         \frac{y_i}{x_i^2 + y_i^2} \sin θ_i,\
         \cos θ_i \right)^T,
     \\
-    θ_i &=&\ \frac{π}{2} \tanh \frac{r_i - r_0}{δ},
+    θ_i &=&\ \frac{π}{2} (\tanh \frac{r_i - r_0}{δ} - 1),
 
 where :math:`x_i` and :math:`y_i` are the :math:`x` and :math:`y` coordinates
 of site :math:`i`, and :math:`r_i = \sqrt{x_i^2 + y_i^2}`.