doc: release: 3.1.0 notes are no longer a working draft

Remove that from the title.

Signed-off-by: Martí Bolívar <[email protected]>

Author: mbolivar-nordic

doc/releases/release-notes-3.1.rst

@@ -2,8 +2,8 @@
 
 .. _zephyr_3.1:
 
-Zephyr 3.1.0 (Working Draft)
-############################
+Zephyr 3.1.0
+############
 
 The following sections provide detailed lists of changes by component.