[doc] fix code-block rendering
Author: erensahin
Pull request: yes

double : prevents code-block section to be rendered, so made it single :

# What does this PR do?

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fixes code-block section rendering in the GPT documentation.


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [ ] Did you write any new necessary tests?


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