Rename tl;dr to High Level Advice

Thanks Pierre Moulon :)

Change-Id: I7e1948efc2fe27d9ddac46f8902ac39677e26eb4

docs/source/derivatives.rst

diff --git a/docs/source/derivatives.rst b/docs/source/derivatives.rst
index e0638c9..808c942 100644
--- a/docs/source/derivatives.rst
+++ b/docs/source/derivatives.rst
@@ -44,8 +44,8 @@
 three approaches in the context of Ceres Solver with sufficient detail
 that the user can make an informed choice.
 
-tl;dr
------
+High Level Advice
+-----------------
 
 And for the impatient amongst you, here is some high level advice: