Rename Trigger Specs header (#1526)

Author: apasel422

index.bs

@@ -708,9 +708,9 @@ A <dfn>trigger-data matching mode</dfn> is one of the following:
 
 </dl>
 
-<h3 id="trigger-specs-header">Trigger specs</h3>
+<h3 dfn-type=dfn>Trigger data set</h3>
 
-A <dfn>trigger data set</dfn> is a [=set=] whose items are unsigned 32-bit
+A trigger data set is a [=set=] whose items are unsigned 32-bit
 integers.
 
 To <dfn>find matching trigger data</dfn> given an [=attribution source=]