Fix URL in report-uri request (#731)

The endpoint variable appears to be unused. Looking at the creation
of the request, I think the intention was to use it in the URL. That's
also what other browsers appear to be doing.

Author: TimvdLippe

index.bs

@@ -1930,7 +1930,7 @@ Content-Type: application/reports+json
                   :   <a for="request">method</a>
                   ::  "`POST`"
                   :   <a for="request">url</a>
-                  ::  |violation|'s <a for="violation">url</a>
+                  ::  |endpoint|
                   :   <a for="request">origin</a>
                   ::  |violation|'s <a for="violation">global object</a>'s <a>relevant settings
                       object</a>'s <a for="environment settings object">origin</a>