Change Notion name to lowercase in marketplace.json

According to the SKILLS spec (see https://agentskills.io/specification#:~:text=Max%2064%20characters.%20Lowercase%20letters%2C%20numbers%2C%20and%20hyphens%20only.%20Must%20not%20start%20or%20end%20with%20a%20hyphen.) names should not contain uppercase letters. This prevents loading the marketplace in spec-compliant agents.

Update the name to be in lowercase.

.claude-plugin/marketplace.json

@@ -505,7 +505,7 @@
       "homepage": "https://github.com/anthropics/claude-plugins-public/tree/main/external_plugins/linear"
     },
     {
-      "name": "Notion",
+      "name": "notion",
       "description": "Notion workspace integration. Search pages, create and update documents, manage databases, and access your team's knowledge base directly from Claude Code for seamless documentation workflows.",
       "category": "productivity",
       "source": {