chore: descriptive comment for path variable (GoogleCloudPlatform#1291)

Author: b-loved-dreamer

texttospeech/src/synthesize_text_file.php

@@ -38,7 +38,7 @@
 use Google\Cloud\TextToSpeech\V1\VoiceSelectionParams;
 
 /** Uncomment and populate these variables in your code */
-// $path = 'Path to file to synthesize';
+// $path = 'The text file to be synthesized. (e.g., hello.txt)';
 
 // create client object
 $client = new TextToSpeechClient();