Fix flag-api typo and clarified instance initialization timing

[HaHaWTH]

Overview

Some corrections to the PlotSquared flag-api doc, and clarified instance initialization timing.

Description

Clarifying the GlobalFlagContainer initialization stage

Submitter Checklist

• Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
• Ensure that the pull request title represents the desired changelog entry.
• New public fields and methods are annotated with @since TODO.
• I read and followed the contribution guidelines.