Fix TURN server example in WebRTC documentation

WebRTC GDNative plugin uses `credential` and not `credentials`.
74f2c78db5/src/WebRTCLibPeerConnection.cpp (L35-L37)

(cherry picked from commit 280496a2c3)

modules/webrtc/doc_classes/WebRTCPeerConnection.xml

@@ -97,7 +97,7 @@
 				        {
 				            "urls": [ "turn:turn.example.com:3478" ], # One or more TURN servers.
 				            "username": "a_username", # Optional username for the TURN server.
-				            "credentials": "a_password", # Optional password for the TURN server.
+				            "credential": "a_password", # Optional password for the TURN server.
 				        }
 				    ]
 				}