Fix typo in a variable name in example code. (#1104)

* Fix typo in a variable name in example code.

* Rename df2dx2 to d2fdx2 - the appropriate naming for the second derivative

* Update CONTRIBUTING.md - add needed python packages, and a virtual-env hint

* Revert "Fix typo in a variable name in example code."

This reverts commit bc10a175345c3e20eab689c47f7d866073a3c233.

* Rename df2dx2 to d2fdx2

examples/cpp/tutorial.cpp

@@ -89,8 +89,8 @@ void automatic_differentiation() {
   // dfdx is 2 * x
 
   // Get the second derivative by composing grad with grad
-  auto df2dx2 = grad(grad(fn))(x);
-  // df2dx2 is 2
+  auto d2fdx2 = grad(grad(fn))(x);
+  // d2fdx2 is 2
 }
 
 int main() {