Update 04.md

docs/1.7/04.md

@@ -76,7 +76,7 @@ b = torch.tensor([6., 4.], requires_grad=True)
 ![](img/tex4-1.gif)
 
 ```py
-Q = 3`a`3 - b**2
+Q = 3*a**3 - b**2
 
 ```
 
@@ -104,7 +104,7 @@ Q.backward(gradient=external_grad)
 
 ```py
 # check if collected gradients are correct
-print(9`a`2 == a.grad)
+print(9*a**2 == a.grad)
 print(-2*b == b.grad)
 
 ```
@@ -233,4 +233,4 @@ optimizer = optim.SGD(model.fc.parameters(), lr=1e-2, momentum=0.9)
 
 [下载 Jupyter 笔记本：`autograd_tutorial.ipynb`](https://pytorch.org/tutorials/_downloads/009cea8b0f40dfcb55e3280f73b06cc2/autograd_tutorial.ipynb)
 
-[由 Sphinx 画廊](https://sphinx-gallery.readthedocs.io)生成的画廊
\ No newline at end of file
+[由 Sphinx 画廊](https://sphinx-gallery.readthedocs.io)生成的画廊