Fix incorrect code-snippet in optimizers doc

[theblackfly]

In the first code block in the optimizers doc training_step(...) should take self as the first argument. It's missing at the moment. It's a simple but necessary fix.

What does this PR do?

Not discussed in Github issues as far as I'm aware. It's a simple typo-fix in one of the code-snippets in the documentation.

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[Borda]

@theblackfly mind reopen the PR :]

[theblackfly]

@Borda An identical PR is now at #7598. Sorry about this.