Update getstarted-about-mainconcepts.rst

[mpchadwick]

Fix document

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• [ ] I've followed the contributing guidelines to the best of my understanding
• [ ] I've read and understood the contributing guidelines
• [ ] I've formatted my code according to Natron's code style
• [ ] I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] My change requires a change to the documentation
  • [ ] I have updated the documentation accordingly

What does this pull request do?

[Your answer, explain the motivation for making this change.]

Show a few screenshots (if this is a visual change)

[Your answer, delete this section if it is not a visual change.]

Have you tested your changes (if applicable)? If so, how?

[Your answer]

Futher details of this pull request

[Your answer]