fixup! Be explicit that 'custom' is an object

Signed-off-by: Joshua Lock <[email protected]>

Author: joshuagl

tuf-spec.md

@@ -834,11 +834,12 @@ repo](https://github.com/theupdateframework/specification/issues).
    the hexdigest of the cryptographic function computed on the target file.
 
    If defined, the elements and values of the CUSTOM object will be made
-   available to the client application.  The format of, and information in,
-   CUSTOM is opaque to the framework and can include version numbers,
-   dependencies, requirements, and any other data that the application wants to
-   include to describe the file at TARGETPATH.  The application may use this
-   information to guide download decisions.
+   available to the client application.  The format of the CUSTOM object is
+   opaque to the framework, which only needs to know that the "custom"
+   attribute maps to an object.  The CUSTOM object may include version
+   numbers, dependencies, requirements, or any other data that the application
+   wants to include to describe the file at TARGETPATH.  The application may
+   use this information to guide download decisions.
 
    DELEGATIONS is an object whose format is the following: