fix docs on data generation for object detection

object_detection/g3doc/preparing_inputs.md

@@ -14,9 +14,9 @@ Extract the tar file and run the `create_pascal_tf_record` script:
 ```
 # From tensorflow/models/object_detection
 tar -xvf VOCtrainval_11-May-2012.tar
-./create_pascal_tf_record --data_dir=VOCdevkit \
+python create_pascal_tf_record.py --data_dir=VOCdevkit \
     --year=VOC2012 --set=train --output_path=pascal_train.record
-./create_pascal_tf_record --data_dir=/home/user/VOCdevkit \
+python create_pascal_tf_record.py --data_dir=/home/user/VOCdevkit \
     --year=VOC2012 --set=val --output_path=pascal_val.record
 ```
 
@@ -36,7 +36,7 @@ file and run the `create_pet_tf_record` script to generate TFRecords.
 # From tensorflow/models/object_detection
 tar -xvf annotations.tar.gz
 tar -xvf images.tar.gz
-./create_pet_tf_record --data_dir=`pwd` --output_dir=`pwd`
+python create_pet_tf_record.py --data_dir=`pwd` --output_dir=`pwd`
 ```
 
 You should end up with two TFRecord files named pet_train.record and