Fix installation command

Signed-off-by: gaelgoth <gothuey.gael@gmail.com>

.changeset/honest-waves-rest.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-mui-to-bui': patch
+---
+
+Fix installation command

plugins/mui-to-bui/README.md

@@ -11,7 +11,7 @@ The Backstage UI Themer helps you convert an existing MUI v5 theme into Backstag
 Run this from your Backstage repo root:
 
 ```bash
-yarn add --cwd packages/app @backstage/plugin-mui-to-bui
+yarn --cwd packages/app add @backstage/plugin-mui-to-bui
 ```
 
 ### 2) Wire it up depending on your frontend system