Replace 'charts' with 'chart'

What does this MR do?

This MR makes a documentation change, replacing the directory being referred to from charts to chart.

Are there points in the code the reviewer needs to double check?

n/a

Why was this MR needed?

This makes it consistent with Auto DevOps' gitlab-ci.yml.

Screenshots (if relevant)

n/a

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

See also: gitlab-ci-yml!114 (closed)