Update networkx_plotting.py

Fixing the error message in self-cycle cases.

Signed-off-by: Ahmet Kapkic <ahmetkapkic@gmail.com>

dowhy/utils/networkx_plotting.py

@@ -42,7 +42,7 @@ def plot_causal_graph_networkx(
         if edge[0] == edge[1]:
             raise ValueError(
                 "Node %s has a self-cycle, i.e. a node pointing to itself. Plotting self-cycles is "
-                "currently only supported for plots using Graphviz! Consider installing the corresponding"
+                "currently only supported for plots using Graphviz! Consider installing the corresponding "
                 "requirements." % edge[0]
             )