docs: fix image url for completion command

Author: JeanMeche

packages/angular/cli/src/commands/completion/long-description.md

@@ -4,7 +4,7 @@ discover and use CLI commands without lots of memorization.
 
 ![A demo of Angular CLI autocompletion in a terminal. The user types several partial `ng` commands,
 using autocompletion to finish several arguments and list contextual options.
-](generated/images/guide/cli/completion.gif)
+](assets/images/guide/cli/completion.gif)
 
 ## Automated setup