First of all, you should not edit the Joomla core files. If you need to change something, template overrides is the way to go. However, in this case it shouldn't be needed.
It looks like you're mixing up the search component (
components/com_search) and the search module (
I assume you're using the module, and you can set the placeholder directly in the module settings. Go to Extensions -> Modules and locate your search module. Open the settings, and edit the field Box text to whatever you want the placeholder text to be.
As you can see from the tooltip, the box will use a string from your language file if the box is left empty. This means you can also create a language override in order to change the text.
In that case the string you want to override is