Development Documentation: fix minor typo

What does this MR do?

In the Development Documentation Styleguide there's a typo in the Do and Don't table of examples.

This MR replaces "that's'" with "that's".

Screenshots

Does this MR meet the acceptance criteria?

I'm not sure if any of the below applies to this change. Would appreciate any directions here.

Conformity

I think N/A.

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

N/A

Security

N/A