fix(readme): correct codex marketplace install command

Signed-off-by: Michael Kantor <6068672+kantorcodes@users.noreply.github.com>

Author: kantorcodes

README.md

@@ -63,13 +63,20 @@ Install plugins directly from this curated list by pointing Codex at the repo ma
 ```bash
 # Add this repo as a marketplace source (one-time setup)
 codex plugin marketplace add \
-  "https://raw.githubusercontent.com/hashgraph-online/awesome-codex-plugins/main/.agents/plugins/marketplace.json"
+  https://github.com/hashgraph-online/awesome-codex-plugins.git \
+  --ref main \
+  --sparse .agents/plugins \
+  --sparse plugins
 
 # Then browse and install (the marketplace name is derived from the repo name)
 codex plugin list --source awesome-codex-plugins
 codex plugin install <plugin-name> --source awesome-codex-plugins
 ```
 
+Do not use the raw `marketplace.json` URL with `codex plugin marketplace add`.
+The Codex marketplace command clones a Git repository, so a raw GitHub file URL is
+treated like a repo URL and fails with `remote: 404: Not Found`.
+
 **Desktop App / IDE Extension:**
 1. Open Codex settings → Plugins → Next to search plugins input click on menu and select → `+Add More...` 
 <img width="1462" height="466" alt="image" src="https://github.com/user-attachments/assets/ae15f505-58a8-4199-bb7b-56a07b670b10" />