fix sparse insertion example
diff --git a/doc/C09_TutorialSparse.dox b/doc/C09_TutorialSparse.dox
index 6022cc5..cf0baa5 100644
--- a/doc/C09_TutorialSparse.dox
+++ b/doc/C09_TutorialSparse.dox
@@ -157,7 +157,7 @@
A typical scenario to insert nonzeros is illustrated bellow:
\code
1: SparseMatrix<double> mat(rows,cols); // default is column major
-2: mat.reserve(VectorXi::Constant(rows,6));
+2: mat.reserve(VectorXi::Constant(cols,6));
3: for each i,j such that v_ij != 0
4: mat.insert(i,j) = v_ij; // alternative: mat.coeffRef(i,j) += v_ij;
5: mat.makeCompressed(); // optional