Documentation: Normalize release note headings

Headings are used to populate release note TOC.

Change-Id: I39b018ed4498555044616a3aa660abe1047b5449
Signed-off-by: Tom Hiller <thrilleratplay@gmail.com>
Reviewed-on: https://review.coreboot.org/27720
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

Documentation/releases/coreboot-4.1-relnotes.md

@@ -1,5 +1,5 @@
-Announcing coreboot 4.1
+coreboot 4.1 release notes
-=======================
+==========================
 
 Dear coreboot community,
 

Documentation/releases/coreboot-4.2-relnotes.md

@@ -1,5 +1,5 @@
-Announcing coreboot 4.2
+coreboot 4.2 release notes
-=======================
+==========================
 
 Halloween 2015 release - just as scary as that sounds
 

Documentation/releases/coreboot-4.3-relnotes.md

@@ -1,5 +1,5 @@
-Announcing coreboot 4.3
+coreboot 4.3 release notes
-=======================
+==========================
 
 The "Oh, has FOSDEM started?" release
 

Documentation/releases/coreboot-4.4-relnotes.md

@@ -1,5 +1,5 @@
-Announcing coreboot 4.4
+coreboot 4.4 release notes
-=======================
+==========================
 
 We are happy to announce the release of coreboot 4.4.  This is our
 fourth quarterly release.  Since the last release, we've had 850 commits

Documentation/releases/coreboot-4.5-relnotes.md

@@ -1,5 +1,5 @@
-Announcing coreboot 4.5
+coreboot 4.5 release notes
-=======================
+==========================
 
 We are happy to announce the release of coreboot 4.5
 

Documentation/releases/coreboot-4.6-relnotes.md

@@ -1,5 +1,5 @@
-Announcing coreboot 4.6
+coreboot 4.6 release notes
-=======================
+==========================
 
 We are happy to announce the April 2017 release of coreboot, version
 4.6.

Documentation/releases/coreboot-4.9-relnotes.md

@@ -1,4 +1,4 @@
-coreboot 4.9 release notes
+Upcoming release - coreboot 4.9
 ==========================
 
 The 4.9 release is planned for November 2018