Change order of optimizer.step() in documentation

[tgilbert09]

Commit be6e86342233bdc932deace52d7388073fbb66b4 changed the order of optimization steps in the "# Backpropagation" section of the code (see lines 161 to 163). This commit updates the order of optimization steps in the description/comments to match the code.

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3006

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