fix: Correct theme command in tui.mdx (#6410)

packages/web/src/content/docs/tui.mdx

@@ -235,7 +235,7 @@ Share current session. [Learn more](/docs/share).
 List available themes.
 
 ```bash frame="none"
-/themes
+/theme
 ```
 
 **Keybind:** `ctrl+x t`