fix typo on NeRF tutorial (#1163)

Summary:
Hello, I'm Seungoh from South Korea.

I'm finding typo while I'm learning tutorials.
Wrong numbers are changed to right numbers.

Thank you.

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/1163

Reviewed By: patricklabatut

Differential Revision: D35546843

Pulled By: bottler

fbshipit-source-id: b6e70cdf821fd4a108dfd416e8f4bcb3ecbeb449

docs/tutorials/fit_simple_neural_radiance_field.ipynb

@@ -815,7 +815,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## 5. Visualizing the optimized neural radiance field\n",
+    "## 6. Visualizing the optimized neural radiance field\n",
     "\n",
     "Finally, we visualize the neural radiance field by rendering from multiple viewpoints that rotate around the volume's y-axis."
    ]
@@ -865,7 +865,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## 6. Conclusion\n",
+    "## 7. Conclusion\n",
     "\n",
     "In this tutorial, we have shown how to optimize an implicit representation of a scene such that the renders of the scene from known viewpoints match the observed images for each viewpoint. The rendering was carried out using the PyTorch3D's implicit function renderer composed of either a `MonteCarloRaysampler` or `NDCMultinomialRaysampler`, and an `EmissionAbsorptionRaymarcher`."
    ]