updated error message with new settings filename

updating help message to reflect that we renamed with:
git mv hadoop-streaming-env.sh settings.sh

mapred_tfidf.py

@@ -94,7 +94,7 @@ def get_output_dir(sub_dir):
     except KeyError as e:
         err_msg = '''
                   ERROR: environment variable NLTK_HOME undefined
-                  have you run "source hadoop-streaming-env.sh"?
+                  have you run "source settings.sh"?
                   '''
         print(err_msg, file=sys.stderr)
         raise e