Update calculus.md

If users are running the code locally in VSCode, the figure won't appear if lack this line of code

chapter_preliminaries/calculus.md

@@ -272,6 +272,7 @@ def plot(X, Y=None, xlabel=None, ylabel=None, legend=[], xlim=None,
     for x, y, fmt in zip(X, Y, fmts):
         axes.plot(x,y,fmt) if len(x) else axes.plot(y,fmt)
     set_axes(axes, xlabel, ylabel, xlim, ylim, xscale, yscale, legend)
+    # d2l.plt.show() # if you are running the code locally and can't see the figure, try add this line of code.
 ```
 
 Now we can [**plot the function $u = f(x)$ and its tangent line $y = 2x - 3$ at $x=1$**],