Fix create-an-app.md output image (#1897)

The image src url is broken.

docs/getting-started/create-an-app.md

@@ -22,7 +22,7 @@ This will create a new Backstage App inside the current folder. The name of the
 app-folder is the name that was provided when prompted.
 
 <p align='center'>
-    <img src='https://github.com/spotify/backstage/raw/master/docs/create-app_output.png' width='600' alt='create app'>
+    <img src='https://github.com/spotify/backstage/raw/master/docs/getting-started/create-app_output.png' width='600' alt='create app'>
 </p>
 
 Inside that directory, it will generate all the files and folder structure