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,