Update DLP redact image code sample region to include mimetype import…

… [(#1928)](GoogleCloudPlatform/python-docs-samples#1928)

In response to feedback where a user was confused that the mimetype
import was missing from the code sample in the documentation.

samples/snippets/redact.py

@@ -18,11 +18,14 @@
 from __future__ import print_function
 
 import argparse
+# [START dlp_redact_image]
 import mimetypes
+# [END dlp_redact_image]
 import os
 
 
 # [START dlp_redact_image]
+
 def redact_image(project, filename, output_filename,
                  info_types, min_likelihood=None, mime_type=None):
     """Uses the Data Loss Prevention API to redact protected data in an image.