Two minor fixes in doc and tutorial (#254)

Summary:
`INSTALL.md` On Windows, pip install from git does not work with single quotes. I replace them with double quote as those should work for any OS.

`camera_position_optimization_with_differentiable_rendering.ipynb` To extract the silhouette from the reference image, we need to reject white values, not black values, because the background of the ref is white:
![ref](https://user-images.githubusercontent.com/5802849/86496925-b72e9100-bd7f-11ea-9f61-72abb3efe1b5.png)

Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/254

Reviewed By: gkioxari

Differential Revision: D23798508

Pulled By: nikhilaravi

fbshipit-source-id: 1fbec291c48c367539a8a4fea389a109aab49e3d

INSTALL.md

@@ -89,11 +89,11 @@ CUDA support will be included if CUDA is available in pytorch or if the environm
 
 ### 1. Install from GitHub
 ```
-pip install 'git+https://github.com/facebookresearch/pytorch3d.git'
+pip install "git+https://github.com/facebookresearch/pytorch3d.git"
 ```
 To install using the code of the released version instead of from the main branch, use the following instead.
 ```
-pip install 'git+https://github.com/facebookresearch/pytorch3d.git@stable'
+pip install "git+https://github.com/facebookresearch/pytorch3d.git@stable"
 ```
 
 For CUDA builds with versions earlier than CUDA 11, set `CUB_HOME` before building as described above.
@@ -101,7 +101,7 @@ For CUDA builds with versions earlier than CUDA 11, set `CUB_HOME` before buildi
 **Install from Github on macOS:**
 Some environment variables should be provided, like this.
 ```
-MACOSX_DEPLOYMENT_TARGET=10.14 CC=clang CXX=clang++ pip install 'git+https://github.com/facebookresearch/pytorch3d.git'
+MACOSX_DEPLOYMENT_TARGET=10.14 CC=clang CXX=clang++ pip install "git+https://github.com/facebookresearch/pytorch3d.git"
 ```
 
 ### 2. Install from a local clone