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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[codecov]

Codecov Report

Merging #7598 (53d0abc) into master (20f6337) will decrease coverage by 5%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #7598    +/-   ##
=======================================
- Coverage      92%     88%    -5%     
=======================================
  Files         197     197            
  Lines       12878   12878            
=======================================
- Hits        11899   11312   -587     
- Misses        979    1566   +587