Correct package documentation

What does this MR do?

doc/user/packages/index.md incorrectly states, "If you cannot find the {package} Packages & Registries > Package Registry entry under your project's sidebar, it is not enabled in your GitLab instance." In fact, that entry will only appear if packages are enabled 1) instance-wide and 2) for the project in question. This MR corrects the error.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Style guides

Closes #217837 (closed)