Fixing wrong type in SparseMatrixView documentation.

       * \tparam Keep is a type of lambda function for storing results of reduction in each row. It is declared as

       *

       * ```

       * auto keep = [=] __cuda_callable__ ( const IndexType rowIdx, const double& value ) { ... };

       * auto keep = [=] __cuda_callable__ ( const IndexType rowIdx, const RealType& value ) { ... };

       * ```

       *

       * \tparam FetchValue is type returned by the Fetch lambda function.