docs: fix typo in extensions command in README (#3096)

### 🛠️ Fix typo in Gemini CLI section

This pull request fixes a typo in the **Gemini CLI** section of the
README.

* Corrected `/exttensions list` → `/extensions list`

### ✅ Why this change?

The incorrect command may confuse users when trying to list extensions
in the CLI.

### 📌 Scope

* Documentation update only
* No code changes

---

Let me know if any changes are needed.

Co-authored-by: Wenxin Du <117315983+duwenxin99@users.noreply.github.com>

Author: NaniToka

README.md

@@ -446,7 +446,7 @@ After installing a [Gemini CLI extensions](https://geminicli.com/extensions/), t
 gemini
 
 # List extensions
-/exttensions list
+/extensions list
 # List MCP servers
 /mcp list
 ```