Update doc/specs/stdlib_sparse.md

Co-authored-by: Federico Perini <federico.perini@gmail.com>

doc/specs/stdlib_sparse.md

@@ -133,7 +133,7 @@ ELL%index(4,1:3) = [1,3,4]
 Experimental
 
 #### Description
-The Sliced ELLPACK format `SELLC` is a variation of the `ELLPACK` format. This modification reduces the storage size compared to the `ELLPACK` format but maintaining its efficient data access scheme. It can be seen as an intermediate format between `CSR` and `ELLPACK`. For more details read [here](https://arxiv.org/pdf/1307.6209v1)
+The Sliced ELLPACK format `SELLC` is a variation of the `ELLPACK` format. This modification reduces the storage size compared to the `ELLPACK` format but maintaining its efficient data access scheme. It can be seen as an intermediate format between `CSR` and `ELLPACK`. For more details read [the reference](https://arxiv.org/pdf/1307.6209v1)
 
 <!-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -->
 ## `spmv` - Sparse Matrix-Vector product