[llvm][Docs] Add explanatory note to MyFirstTypoFix intro

Summary:
To make it 100% clear that the changes are an example and the guide
should later be used with the reader's own changes to submit an
actual PR.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251496

Author: DavidSpickett

llvm/docs/MyFirstTypoFix.rst

@@ -9,8 +9,14 @@ Introduction
 ============
 
 This tutorial will guide you through the process of making a change to
-LLVM, and contributing it back to the LLVM project. We'll be making a
-change to Clang, but the steps for other parts of LLVM are the same.
+LLVM, and contributing it back to the LLVM project.
+
+.. note::
+   The code changes presented here are only an example and not something you
+   should actually submit to the LLVM project. For your first real change to LLVM,
+   the code will be different but the rest of the guide will still apply.
+
+We'll be making a change to Clang, but the steps for other parts of LLVM are the same.
 Even though the change we'll be making is simple, we're going to cover
 steps like building LLVM, running the tests, and code review. This is
 good practice, and you'll be prepared for making larger changes.