Avoid swapping of indices in v3 track proxy covariance()

Event/TrackEvent/include/Event/Track_v3.h

@@ -337,20 +337,19 @@ namespace LHCb::Event::v3 {
         auto const&                      stateCov = this->template field<Tag::StateCovs>()[state_index( StateLoc )];
         LHCb::LinAlg::MatSym<float_v, 5> cov; // This is not zero-initialised
         cov( 0, 0 ) = stateCov.x_x();
-        // FIXME: this is always calling swap
-        cov( 0, 1 ) = stateCov.x_y();
-        cov( 0, 2 ) = stateCov.x_tx();
-        cov( 0, 3 ) = stateCov.x_ty();
-        cov( 0, 4 ) = stateCov.x_QoverP();
+        cov( 1, 0 ) = stateCov.x_y();
+        cov( 2, 0 ) = stateCov.x_tx();
+        cov( 3, 0 ) = stateCov.x_ty();
+        cov( 4, 0 ) = stateCov.x_QoverP();
         cov( 1, 1 ) = stateCov.y_y();
-        cov( 1, 2 ) = stateCov.y_tx();
-        cov( 1, 3 ) = stateCov.y_ty();
-        cov( 1, 4 ) = stateCov.y_QoverP();
+        cov( 2, 1 ) = stateCov.y_tx();
+        cov( 3, 1 ) = stateCov.y_ty();
+        cov( 4, 1 ) = stateCov.y_QoverP();
         cov( 2, 2 ) = stateCov.tx_tx();
-        cov( 2, 3 ) = stateCov.tx_ty();
-        cov( 2, 4 ) = stateCov.tx_QoverP();
+        cov( 3, 2 ) = stateCov.tx_ty();
+        cov( 4, 2 ) = stateCov.tx_QoverP();
         cov( 3, 3 ) = stateCov.ty_ty();
-        cov( 3, 4 ) = stateCov.ty_QoverP();
+        cov( 4, 3 ) = stateCov.ty_QoverP();
         cov( 4, 4 ) = stateCov.QoverP_QoverP();
         return cov;
       }