Update tuf-spec.md

Co-authored-by: Joshua Lock <[email protected]>

Author: mnm678

tuf-spec.md

@@ -411,7 +411,7 @@ repo](https://github.com/theupdateframework/specification/issues).
  + **3.1.2 Metadata files**
 
    The filenames and directory structure of repository metadata are strictly
-   defined.  All metadata filenames will have an extension based on the
+   defined.  All metadata filenames will have an extension (EXT) based on the
    metaformat, for example json metadata files would have an EXT of json.
    The following are the metadata files of top-level roles relative
    to the base URL of metadata available from a given repository mirror.