tensor_illustration with correct numbers and better fonts for README …

…file (pytorch#20751)

Summary:
Fix of README tensor image for issue pytorch#20641
Numbers are fixed, symbols made more readable.
Pull Request resolved: pytorch#20751

Differential Revision: D15495706

Pulled By: ezyang

fbshipit-source-id: b6013574d16253ec681fc57143efe3d53952fbe9

README.md

@@ -55,7 +55,7 @@ Elaborating further:
 
 If you use NumPy, then you have used Tensors (a.k.a ndarray).
 
-![Tensor illustration](https://github.com/pytorch/pytorch/blob/master/docs/source/_static/img/tensor_illustration.png)
+![Tensor illustration](./docs/source/_static/img/tensor_illustration.png)
 
 PyTorch provides Tensors that can live either on the CPU or the GPU, and accelerates the
 computation by a huge amount.