Correct file extention of README (#10125)

diff --git a/examples/addressbook.proto b/examples/addressbook.proto
index 1bff4ad..5f52698 100644
--- a/examples/addressbook.proto
+++ b/examples/addressbook.proto
@@ -1,4 +1,4 @@
-// See README.txt for information and build instructions.
+// See README.md for information and build instructions.
 //
 // Note: START and END tags are used in comments to define sections used in
 // tutorials.  They are not part of the syntax for Protocol Buffers.