Update messages/webapp.dev.md

Co-authored-by: Juliet Shackell <63259011+jshackell-sfdc@users.noreply.github.com>

Author: deepu-mungamuri94

messages/webapp.dev.md

@@ -178,7 +178,7 @@ Start your dev server manually at that URL, or add dev.command to webapplication
 
 # error.port-in-use
 
-Port %s is already in use. Try a different port with --port or stop the service using that port.
+Port %s is already in use. Try specifying a different port with the --port flag or stopping the service that's using the port.
 
 # error.dev-server-failed