Update README to specify metrics were computed on the standard COCO val2017 validation set

README.md

@@ -57,7 +57,7 @@ The model achieves the following metrics on the COCO person validation set:
 | Mask Precision      | 0.843 |
 | Mask Recall         | 0.723 |
 
-These metrics were computed on a validation set of 5,000 images with 10,777 instances.
+These metrics were computed on the standard COCO `val2017` validation set.
 
 ## Example Results