MOBILE-1701 quiz: Add missing string continuepreview

Author: dpalou

www/addons/mod/quiz/lang/en.json

@@ -14,6 +14,7 @@
 	"confirmstartheader": "Timed quiz",
 	"connectionerror": "Network connection lost. (Autosave failed).\n\nMake a note of any responses entered on this page in the last few minutes, then try to re-connect.\n\nOnce connection has been re-established, your responses should be saved and this message will disappear.",
 	"continueattemptquiz": "Continue the last attempt",
+	"continuepreview": "Continue the last preview",
 	"errordownloading": "Error downloading required data.",
 	"errorgetattempt": "Error getting attempt data.",
 	"errorgetquestions": "Error getting questions.",