Update definitions from 'todo' to 'to-do'

What does this MR do?

Updates some API definitions to use to-do (correct) instead of todo (incorrect). Sticking to user-facing API definitions only—no other changes—todo is valid in code and it should stay that way. I just want the spelling issues in the documentation to stop.

!52951 (merged) is related, and tackles more words.

Related to #230890 (closed) which other people were working on at the same time.

Related to #276206 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team