Remove substructure-specific extension wording (#481)

Resolves #478

specification/gedcom-1-hierarchical-container-format.md

@@ -457,7 +457,7 @@ defines the following tags
 Note that at the time of writing, the [FOAF](https://xmlns.com/foaf/spec/20140114.html) URIs used in this example are not URLs.
 :::
 
-The meaning of a documented extension tag is identified by its superstructure type and its URI, not its tag.
+The meaning of a documented extension tag is identified by its URI, not its tag.
 As such each documented extension tag needs its own URI: it is its URI, not its tag, that defines its meaning.
 Documented extension tags can be changed freely by modifying the schema,
 though it is recommended that documented extension tags not be changed.