doc: process.rst: Use subsubheading for "Phases of the Development Process"

These sections which talk about the different phases of the development
process should be using the subsubheading identifier.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>

doc/develop/process.rst

@@ -34,7 +34,7 @@ It is followed by a *Stabilization Period*.
 The end of a Release Cycle is marked by the release of a new U-Boot version.
 
 Merge Window
-------------
+^^^^^^^^^^^^
 
 The Merge Window is the period when new patches get submitted (and hopefully
 accepted) for inclusion into U-Boot mainline. This period lasts for 21 days (3
@@ -44,7 +44,7 @@ This is the only time when new code (like support for new processors or new
 boards, or other new features or reorganization of code) is accepted.
 
 Twilight Time
--------------
+^^^^^^^^^^^^^
 
 Usually patches do not get accepted as they are - the peer review that takes
 place will usually require changes and resubmissions of the patches before they
@@ -65,13 +65,13 @@ the Merge Window does not preclude patches that were already posted from being
 merged for the upcoming release.
 
 Stabilization Period
---------------------
+^^^^^^^^^^^^^^^^^^^^
 
 During the Stabilization Period only patches containing bug fixes get
 applied.
 
 Corner Cases
-------------
+^^^^^^^^^^^^
 
 Sometimes it is not clear if a patch contains a bug fix or not.
 For example, changes that remove dead code, unused macros etc. or