Error in tensor shape of docstring (#11206)

Corrected the tensor shape in the doc string.
The incoming masks are stacked in dim=0 therefore the doc is wrong

Signed-off-by: Jonas Heinle <jonasheinle@googlemail.com>

utils/segment/general.py

@@ -10,7 +10,7 @@ def crop_mask(masks, boxes):
     Vectorized by Chong (thanks Chong).
 
     Args:
-        - masks should be a size [h, w, n] tensor of masks
+        - masks should be a size [n, h, w] tensor of masks
         - boxes should be a size [n, 4] tensor of bbox coords in relative point form
     """