Update documentation getting_started.rst

[oswald1337]

Change O to S in documentation of source text.

Before submitting

• [x] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
• [x] Did you read the contributor guideline?
• [x] Did you make sure to update the docs?
• [x] Did you write any new necessary tests?

What does this PR do?

Fixes a Typo in the documentation.

PR review

Anyone in the community is free to review the PR once the tests have passed. If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃