Update optim.rst for better understanding (#45944)

Summary:
The `i` variable in `Line 272` may cause ambiguity in understanding. I think it should be named as `epoch` variable.

Fixes #{issue number}

Pull Request resolved: #45944

Reviewed By: agolynski

Differential Revision: D24219486

Pulled By: vincentqb

fbshipit-source-id: 2af0408594613e82a1a1b63971650cabde2b576e

docs/source/optim.rst

@@ -269,7 +269,7 @@ and start to collect SWA averages of the parameters at epoch 160:
 >>>           optimizer.zero_grad()
 >>>           loss_fn(model(input), target).backward()
 >>>           optimizer.step()
->>>       if i > swa_start:
+>>>       if epoch > swa_start:
 >>>           swa_model.update_parameters(model)
 >>>           swa_scheduler.step()
 >>>       else: