fix errors in docstrings; use code directive

src/operator/contrib/dgl_graph.cc

@@ -252,7 +252,7 @@ of CSR matrices if return_mapping is True: the first matrix contains edges
 with new edge Ids, the second matrix contains edges with the original
 edge Ids.
 
-Example::
+Example:
 
    .. code:: python
 
@@ -435,7 +435,7 @@ stored in a CSR matrix (the value of the CSR stores the edge Id of the graph).
 output[i] = input[u[i], v[i]] if there is an edge between u[i] and v[i]],
 otherwise output[i] will be -1. Both u and v should be 1D vectors.
 
-Example::
+Example:
 
    .. code:: python
 
@@ -510,7 +510,7 @@ NNVM_REGISTER_OP(_contrib_dgl_adjacency)
 to an adjacency matrix whose values are ones. The output CSR matrix always has
 the data value of float32.
 
-Example::
+Example:
 
    .. code:: python