Add doctest for add_vertex in GraphAdjacencyList. Contributes to #9943 (#13143)

* Add doctest for add_vertex in GraphAdjacencyList. Contributes to #9943 * Update graph_adjacency_list.py --------- Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>

Author: michaelmccamy

graphs/graph_adjacency_list.py

@@ -61,6 +61,15 @@ def add_vertex(self, vertex: T) -> None:
  """
  Adds a vertex to the graph. If the given vertex already exists,
  a ValueError will be thrown.
+
+ >>> g = GraphAdjacencyList(vertices=[], edges=[], directed=False)
+ >>> g.add_vertex("A")
+ >>> g.adj_list
+ {'A': []}
+ >>> g.add_vertex("A")
+ Traceback (most recent call last):
+ ...
+ ValueError: Incorrect input: A is already in the graph.
  """
  if self.contains_vertex(vertex):
  msg = f"Incorrect input: {vertex} is already in the graph."