Correct file extention of README (#10125)

diff --git a/examples/add_person.py b/examples/add_person.py
index aa0fbca..aa53d58 100755
--- a/examples/add_person.py
+++ b/examples/add_person.py
@@ -1,6 +1,6 @@
 #! /usr/bin/env python
 
-# See README.txt for information and build instructions.
+# See README.md for information and build instructions.
 
 import addressbook_pb2
 import sys