[1.14] Fix missing $socketFile variable in Persistent Browser example

[stevebauman]

The current example script references a $socketFile variable that does't exist:

chrome/README.md

Lines 193 to 212 in 9dce1e4

	```php
	use \HeadlessChromium\BrowserFactory;
	use \HeadlessChromium\Exception\BrowserConnectionFailed;
	// path to the file to store websocket's uri
	$socket = \file_get_contents('/tmp/chrome-php-demo-socket');
	try {
	$browser = BrowserFactory::connectToBrowser($socket);
	} catch (BrowserConnectionFailed $e) {
	// The browser was probably closed, start it again
	$factory = new BrowserFactory();
	$browser = $factory->createBrowser([
	'keepAlive' => true,
	]);
	// save the uri to be able to connect again to browser
	\file_put_contents($socketFile, $browser->getSocketUri(), LOCK_EX);
	}
	```