Update ResizeImageServlet.java

The regex expression for path.Split("/") should be "\\\\", otherwise will the following exception occurred.

[19/10/13 22:08:09:765 CST] ERROR image.ResizeImageServlet: 3
java.lang.ArrayIndexOutOfBoundsException: 3
	at com.dotmarketing.servlets.image.ResizeImageServlet.service(ResizeImageServlet.java:107)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)