[fbsync] Fix error message typo (#6682)

Reviewed By: datumbox

Differential Revision: D40138742

fbshipit-source-id: 0b1e2571057e4b3b140f4b898c52c383fd43cb05

Author: NicolasHug

torchvision/models/detection/roi_heads.py

@@ -787,7 +787,7 @@ def forward(
             mask_proposals = [p["boxes"] for p in result]
             if self.training:
                 if matched_idxs is None:
-                    raise ValueError("if in trainning, matched_idxs should not be None")
+                    raise ValueError("if in training, matched_idxs should not be None")
 
                 # during training, only focus on positive boxes
                 num_images = len(proposals)