fix a typo

[STRRL]

plugin -> plugins

it takes me a little time to find that there is a typo in readme

❯ claude plugin install ralph-wiggum@claude-plugin-directory
Installing plugin "ralph-wiggum@claude-plugin-directory"...
✘ Failed to install plugin "ralph-wiggum@claude-plugin-directory": Plugin "ralph-wiggum" not found in marketplace "claude-plugin-directory"

[tobinsouth]

Closing — #102 has the correct fix (the marketplace name is claude-plugins-official, not claude-plugins-directory).