fix typo in block indexing, 3x3 covariance for Pose2 should have just 1x1 block for theta

Author: johnwlambert

gtsam/sfm/ShonanAveraging.cpp

@@ -955,7 +955,7 @@ static BinaryMeasurement<Rot2> convertPose2ToBinaryMeasurementRot2(
         "parseMeasurements<Rot2> can only convert Pose2 measurements "
         "with Gaussian noise models.");
   const Matrix3 M = gaussian->covariance();
-  auto model = noiseModel::Gaussian::Covariance(M.block<2, 2>(2, 2));
+  auto model = noiseModel::Gaussian::Covariance(M.block<1, 1>(2, 2));
   return BinaryMeasurement<Rot2>(f->key1(), f->key2(), f->measured().rotation(),
                                  model);
 }