cmd/gobind: fix a typo that is missing quote at end

Change-Id: I91ffbda9c8f562bc66b75ca8a6360196d41c1b28
Reviewed-on: https://go-review.googlesource.com/74610
Reviewed-by: Elias Naur <elias.naur@gmail.com>

cmd/gobind/doc.go

@@ -204,7 +204,7 @@ to import java.lang.System and call the static method currentTimeMillis:
 
 Similarly, to import NSDate and call the static method [NSDate date]:
 
-	import "ObjC/Foundation/NSDate
+	import "ObjC/Foundation/NSDate"
 
 	d := NSDate.Date()