Fix latex equation in the PEM doc.

Author: Alexander Gorban

docs/pose_estimation_metric.md

@@ -78,10 +78,12 @@ $$\textbf{PEM}(Y,\hat{Y}) = \frac{\sum_{i\in M}\left\|y_{i} -
 
 where $M$ - a set of indices of matched keypoints, $U$ - a set of indices of
 unmatched keypoints (ground truth keypoints without matching predicted keypoints
-or predicted keypoints for unmatched objects); Sets $$Y= \left\{y_i\right\}_{i
-\in M}$ and $\hat{Y} = \left\{\hat{y}_i\right\}_{i \in M}$$ are ground truth
-and predicted 3D coordinates of keypoints; $C=0.25$ - a constant penalty for
-an unmatched keypoint.
+or predicted keypoints for unmatched objects); Sets 
+$`Y = \left\lbrace y_i\right\rbrace_{i \in M}`$
+and
+$`\hat{Y} = \left\lbrace\hat{y}_i\right\rbrace_{i \in M}`$
+are ground truth and predicted 3D coordinates of keypoints; $C=0.25$ - a
+constant penalty for an unmatched keypoint.
 
 
 ## Object Matching Algorithm
@@ -165,4 +167,4 @@ responsible for removing duplicate detections.
 NOTE: The WOD library also implements the [`CppMatcher`](src/waymo_open_dataset/metrics/python/keypoint_metrics.py)
 which maximizes total Intersection over Union (IoU) between predicted and ground
 truth boxes. However, this matcher requires all predictions to have bounding
-boxes and is provided only as a reference.
+boxes and is provided only as a reference.