Add missing ExternalModule import (#292)

# Motivation

The code example is missing this import.

# Content

Add missing import to code example.

# Testing

Tested the code in a codegen script.

# Please check the following before marking your PR as ready for review

- [X] I have added tests for my changes
- [X] I have updated the documentation or added new documentation as
needed

Author: jmvldz

docs/building-with-codegen/traversing-the-call-graph.mdx

@@ -32,6 +32,7 @@ Here's how to build a directed graph of function calls using NetworkX:
 import networkx as nx
 from codegen.sdk.core.interfaces.callable import FunctionCallDefinition
 from codegen.sdk.core.function import Function
+from codegen.sdk.core.external_module import ExternalModule
 
 def create_call_graph(start_func, end_func, max_depth=5):
     G = nx.DiGraph()