Fix indentation level of "Review Criteria" sections of contributor doc

The "Review Criteria" has several sub sections, but they are all on
the same level. Increase the indentation so that they are listed as
sub sections of "Review Criteria" in the table of contents.

Change-Id: I67efaaf13697fa39fc4e11ca56001420dad9e739

Documentation/dev-contributing.txt

@@ -67,7 +67,7 @@ spans, we really do want your code.
 
 
 [[commit-message]]
-== Commit Message
+=== Commit Message
 
 It is essential to have a good commit message if you want your
 change to be reviewed.
@@ -143,7 +143,7 @@ link:https://gerrit-review.googlesource.com/#/settings/http-password[HTTP
 Password tab of the user settings page].
 
 
-== Style
+=== Style
 
 The basic coding style is covered by the tools/GoogleFormat.xml
 doc, see the link:dev-eclipse.html#Formatting[Eclipse Setup]
@@ -197,7 +197,7 @@ Never:
   be removed
   * method parameters: similar to local variables
 
-== Code Organization
+=== Code Organization
 
 Do your best to organize classes and methods in a logical way.
 Here are some guidelines that Gerrit uses:
@@ -235,7 +235,7 @@ Naturally new classes are a little harder; you may want to come
 back and consult this section when creating them.
 
 
-== Design
+=== Design
 
 Here are some design level objectives that you should keep in mind
 when coding:
@@ -267,12 +267,12 @@ when coding:
   * ...and so is Guava (previously known as Google Collections).
 
 
-== Tests
+=== Tests
 
   * Tests for new code will greatly help your change get approved.
 
 
-== Change Size/Number of Files Touched
+=== Change Size/Number of Files Touched
 
 And finally, I probably cannot say enough about change sizes.
 Generally, smaller is better, hopefully within reason.  Do try to