misc: remove quotation marks from TURN configuration (#1111)

Fixes: #1110

env.example

@@ -358,7 +358,7 @@ JIBRI_STRIP_DOMAIN_JID=muc
 JIBRI_LOGS_DIR=/config/logs
 
 # Configure an external TURN server
-# TURN_CREDENTIALS="secret"
+# TURN_CREDENTIALS=secret
 # TURN_HOST=turnserver.example.com
 # TURN_PORT=443
 # TURNS_HOST=turnserver.example.com