[MINOR] Fix minor mistake in MNC sparsity estimation for row bind operations

The wrong variable was used, h1 instead of h2.

Author: ywcb00

src/main/java/org/apache/sysds/hops/estim/EstimatorMatrixHistogram.java

@@ -386,7 +386,7 @@ public static DataCharacteristics deriveOutputCharacteristics(MatrixHistogram h1
 					return new MatrixCharacteristics(h1.getRows(), h1.getCols(),
 						OptimizerUtils.getNnz(h1.getRows(), h1.getCols(), spOut));
 				case RBIND:
-					return new MatrixCharacteristics(h1.getRows()+h1.getRows(), h1.getCols(),
+					return new MatrixCharacteristics(h1.getRows()+h2.getRows(), h1.getCols(),
 						OptimizerUtils.getNnz(h1.getRows()+h2.getRows(), h1.getCols(), spOut));
 				case CBIND:
 					return new MatrixCharacteristics(h1.getRows(), h1.getCols()+h2.getCols(),