Fix error in the "Assign and Unassign Labels" section of the docs.

What does this MR do?

Fixes an error in https://docs.gitlab.com/ee/user/project/labels.html#assign-and-unassign-labels

Currently, the docs suggest applying a label with the /assign quick action which is not correct

You can also assign a label with the /assign @username quick action.

The docs should be updated to suggest using the /label quick action

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