Minor fix in HarmonicEmbedding docstring (#13)

Summary:
The multiplicative factors in function embeddings go from `2**0` to `2**(self.n_harmonic_functions-1)`, and not from `2**0` to `2**self.n_harmonic_functions`.

Pull Request resolved: fairinternal/pytorch3d#13

Reviewed By: nikhilaravi

Differential Revision: D28637894

Pulled By: ignacio-rocco

fbshipit-source-id: da20f39eba9aaa09af5b24be1554a3bfd7556281

Author: ignacio-rocco

docs/tutorials/fit_simple_neural_radiance_field.ipynb

@@ -257,12 +257,12 @@
     "                sin(2*x[..., i]),\n",
     "                sin(4*x[..., i]),\n",
     "                ...\n",
-    "                sin(2**self.n_harmonic_functions * x[..., i]),\n",
+    "                sin(2**(self.n_harmonic_functions-1) * x[..., i]),\n",
     "                cos(x[..., i]),\n",
     "                cos(2*x[..., i]),\n",
     "                cos(4*x[..., i]),\n",
     "                ...\n",
-    "                cos(2**self.n_harmonic_functions * x[..., i])\n",
+    "                cos(2**(self.n_harmonic_functions-1) * x[..., i])\n",
     "            ]\n",
     "            \n",
     "        Note that `x` is also premultiplied by `omega0` before\n",