Fix minor typo in quickstart.md (google#712)

Co-authored-by: Kristopher Overholt <koverholt@gmail.com>

Author: slilichenko

docs/get-started/quickstart.md

@@ -308,7 +308,7 @@ agent will be unable to function.
             piping text to the command like so:
 
             ```shell
-            "Please start by listing files" | adk run file_listing_agent
+            echo "Please start by listing files" | adk run file_listing_agent
             ```
             
         Run the following command, to chat with your Weather agent.