Fix header of "Pitfalls" subsection of Private Changes documentation

Change-Id: I3dacba7ffd910122cf1dbd123276c6ef29fe5f14

Documentation/intro-user.txt

diff --git a/Documentation/intro-user.txt b/Documentation/intro-user.txt
index e502cdd..6b5b543 100644
--- a/Documentation/intro-user.txt
+++ b/Documentation/intro-user.txt
@@ -605,8 +605,7 @@
 exposing secret details.
 
 [[private-changes-pitfalls]]
-Pitfalls
-===
+=== Pitfalls
 
 If private changes are used, be aware of the following pitfalls: