ARI playback: Rename ARI Playback to Playbacks

Before playback was the only non plural resource.  It has been renamed to
playbacks for consistency.

(closes issue ASTERISK-22737)
Reported by: Paul Belanger
........

Merged revisions 402560 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@402562 65c4cc65-6c06-0410-ace0-fbb531ad65f3

rest-api/api-docs/playbacks.json

@@ -5,10 +5,10 @@
 	"apiVersion": "0.0.1",
 	"swaggerVersion": "1.1",
 	"basePath": "http://localhost:8088/stasis",
-	"resourcePath": "/api-docs/playback.{format}",
+	"resourcePath": "/api-docs/playbacks.{format}",
 	"apis": [
 		{
-			"path": "/playback/{playbackId}",
+			"path": "/playbacks/{playbackId}",
 			"description": "Control object for a playback operation.",
 			"operations": [
 				{
@@ -58,7 +58,7 @@
 			]
 		},
 		{
-			"path": "/playback/{playbackId}/control",
+			"path": "/playbacks/{playbackId}/control",
 			"description": "Control object for a playback operation.",
 			"operations": [
 				{