docs: fix Claude Code MCP installation command syntax (supabase#36570)

Add missing -- separator in claude mcp add command. Without this separator,
the CLI incorrectly interprets 'npx' as an environment variable instead of
the command to execute, causing installation to fail.

Fixes the command:
claude mcp add supabase -s local -e SUPABASE_ACCESS_TOKEN=token npx -y @supabase/mcp-server-supabase@latest

To work correctly as:
claude mcp add supabase -s local -e SUPABASE_ACCESS_TOKEN=token -- npx -y @supabase/mcp-server-supabase@latest

Author: vanzan01

apps/docs/content/guides/getting-started/mcp.mdx

@@ -117,7 +117,7 @@ You can also add the Supabase MCP server as a locally-scoped server, which will
 1. Run the following command in your terminal:
 
    ```bash
-   claude mcp add supabase -s local -e SUPABASE_ACCESS_TOKEN=your_token_here npx -y @supabase/mcp-server-supabase@latest
+   claude mcp add supabase -s local -e SUPABASE_ACCESS_TOKEN=your_token_here -- npx -y @supabase/mcp-server-supabase@latest
    ```
 
 Locally-scoped servers take precedence over project-scoped servers with the same name and are stored in your project-specific user settings.