[racket] Where is "2htdp/web-io.rkt" ?

Here http://www.ccs.neu.edu/home/matthias/HtDP2e/i2-3.html  i found this:"You can use the function show-in-browser from "2htdp/web-io.rkt" to display the result of functions such as my-first-web-page in a web browser". Unfortunately, I cannot find web-io.rkt nowhere in Racket, version 5.1.3. Am I doing something wrong?  		 	   		  
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