content: draft: Clarify name of Source L3

Updating name of Source Level 3 to make it more clear by
removing the somewhat ambiguous 'Source Provenance' and
including the language from PR slsa-framework#1143 instead.

Signed-off-by: Tom Hennen <tomhennen@google.com>

docs/spec/draft/source-requirements.md

@@ -100,7 +100,7 @@ All organizations of any size producing software of any kind.
 Benefits:
 Allows source consumers to track changes to the software over time and attribute those changes to the people that made them.
 
-### Level 3: Source Provenance Attestations
+### Level 3: Authenticatable and auditable attestations
 
 Summary:
 A consumer can ask the SCS for everything it knows about a specific revision of a repository.