Update 'bbox_head' to align with new config files

In the new version of the config files, the 'bbox_head' dict is inside 'roi_head'. If one follows the old documentation, it results in this following issue - 

open-mmlab#607

This way 'bbox_head' is not repeated and the error is solved.

docs/en/tutorials/customize_dataset.md

@@ -71,10 +71,10 @@ data = dict(
 
 # 2. model settings
 model = dict(
-    bbox_head=dict(
-        type='RotatedRetinaHead',
-        # explicitly over-write all the `num_classes` field from default 15 to 5.
-        num_classes=15))
+    roi_head=dict(
+        bbox_head=dict(
+            # explicitly over-write all the `num_classes` field from default 15 to 5.
+            num_classes=5)))
 ```
 
 #### 2. Check the annotations of the customized dataset