remove doc gen warning.

doc/library/sparse/index.txt

@@ -49,8 +49,8 @@ grad?
     - performs the true dot without special semantics.
     - dot(sparse, dense), dot(dense, sparse), dot(sparse, sparse)
     - When the operation has the form dot(csr_matrix, dense) the gradient of
-    this operation can be performed inplace by UsmmCscDense. This leads to
-    significant speed-ups.
+      this operation can be performed inplace by UsmmCscDense. This leads to
+      significant speed-ups.
 
 Subtensor selection (aka. square-bracket notation, aka indexing) is not implemented, but the
 CSR and CSC datastructures support effecient implementations.