Add missing import for code sample [(#1898)](GoogleCloudPlatform/python-docs-samples#1898)

Author: anguillanneuf

samples/snippets/transcribe_enhanced_model.py

@@ -28,7 +28,10 @@
 def transcribe_file_with_enhanced_model(path):
     """Transcribe the given audio file using an enhanced model."""
     # [START speech_transcribe_enhanced_model]
+    import io 
+    
     from google.cloud import speech
+    
     client = speech.SpeechClient()
 
     # path = 'resources/commercial_mono.wav'