doc: add stability class to legacy status description

PR-URL: nodejs#42525
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

doc/api/documentation.md

@@ -40,7 +40,7 @@ The stability indices are as follows:
 
 <!-- separator -->
 
-> Stability 3 - Legacy. Although this feature is unlikely to be removed and is
+> Stability: 3 - Legacy. Although this feature is unlikely to be removed and is
 > still covered by semantic versioning guarantees, it is no longer actively
 > maintained, and other alternatives are available.