Fix TURN server example in WebRTC documentation

WebRTC GDNative plugin uses `credential` and not `credentials`.
https://github.com/godotengine/webrtc-native/blob/74f2c78db5cdffa5b2b6ba9cd041061d7694400c/src/WebRTCLibPeerConnection.cpp#L35-L37

(cherry picked from commit 280496a2c3e4d479b01851ddc27e0d5b1d34d7d9)

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.
 				        }
 				    ]
 				}