Update mapper_pipeline.py
Update mapper_pipeline.py

Add a comment for base_path
GoogleCloudPlatform/appengine-mapreduce,mikelambert/appengine-mapreduce,GoogleCloudPlatform/appengine-mapreduce,VirusTotal/appengine-mapreduce,mikelambert/appengine-mapreduce,mikelambert/appengine-mapreduce,mikelambert/appengine-mapreduce,VirusTotal/appengine-mapreduce,VirusTotal/appengine-mapreduce,GoogleCloudPlatform/appengine-mapreduce,VirusTotal/appengine-mapreduce,VirusTotal/appengine-mapreduce,GoogleCloudPlatform/appengine-mapreduce,GoogleCloudPlatform/appengine-mapreduce,mikelambert/appengine-mapreduce