Fix LG spectrum amplitude:

Replace theta with its sine. This is the correctly transformed version from a
Gaussian distribution in 2D k-space and corresponds to the already fixed implementation in meep.

beam_profiles.ipynb

@@ -147,7 +147,7 @@
     "The spectrum amplitude for Laguerre-Gaussian beams taken from eq. (2.22) in  [[Bliokh2013](#Bliokh2013)] is given by\n",
     "\n",
     "\\begin{equation*}\n",
-    "f_\\text{LG}(\\theta,\\phi) = \\theta^{|m|} \\exp\\biggl[-\\Bigl(\\frac{kw_0}{2}\\theta\\Bigr)^2 \\biggr] \\exp(\\mathrm{i} m \\phi)\n",
+    "f_\\text{LG}(\\theta,\\phi) = \\theta^{|m|} \\exp\\biggl[-\\Bigl(\\frac{kw_0}{2}\\sin\\theta\\Bigr)^2 \\biggr] \\exp(\\mathrm{i} m \\phi)\n",
     "\\end{equation*}\n",
     "\n",
     "with azimuthal angle $\\phi$, polar angle $\\theta$ and vortex charge $m$."