Export dfn of 'visible to ReportingObserver'

Since it is marked as being useful for other specs (and is indeed being used in https://w3c.github.io/network-error-logging/ )

Author: dontcallmedom

index.src.html

@@ -241,7 +241,7 @@ <h4 id="concept-report-type">Report Type</h4>
   of data that is contained in the [=report/body=] of a <a>report</a>.
 
   When a <a>report type</a> is defined (in this spec or others), it can be
-  specified to be <dfn>visible to <code>ReportingObserver</code>s</dfn>, meaning
+  specified to be <dfn export>visible to <code>ReportingObserver</code>s</dfn>, meaning
   that <a>reports</a> of that type can be observed by a <a>reporting
   observer</a>. By default, <a>report types</a> are not <a>visible to
   <code>ReportingObserver</code>s</a>.