Correct file extention of README (#10125)

Author: vinzid

examples/AddPerson.java

@@ -1,4 +1,4 @@
-// See README.txt for information and build instructions.
+// See README.md for information and build instructions.
 
 import com.example.tutorial.protos.AddressBook;
 import com.example.tutorial.protos.Person;

examples/ListPeople.java

@@ -1,4 +1,4 @@
-// See README.txt for information and build instructions.
+// See README.md for information and build instructions.
 
 import com.example.tutorial.protos.AddressBook;
 import com.example.tutorial.protos.Person;

examples/Makefile

@@ -1,4 +1,4 @@
-# See README.txt.
+# See README.md.
 
 .PHONY: all cpp java python clean
 

examples/add_person.cc

@@ -1,4 +1,4 @@
-// See README.txt for information and build instructions.
+// See README.md for information and build instructions.
 
 #include <ctime>
 #include <fstream>

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

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.

examples/list_people.cc

@@ -1,4 +1,4 @@
-// See README.txt for information and build instructions.
+// See README.md for information and build instructions.
 
 #include <fstream>
 #include <google/protobuf/util/time_util.h>

examples/list_people.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.
 
 from __future__ import print_function
 import addressbook_pb2