content: draft: reliable -> authenticatable and auditable (slsa-frame…

…work#1143)

Clarify what type of data source level 3 provides to policy enforcement
tools.

'authenticatable and auditable' is more easily understood by the
community than 'reliable'. It's also less prone to misinterpretation
than 'verifiable' which might sound like a much more thorough process
has been done.

fixes slsa-framework#1137

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

docs/spec/draft/source-requirements.md

@@ -110,7 +110,7 @@ Intended for:
 Organizations that want strong guarantees and auditability of their change management processes.
 
 Benefits:
-Provides reliable information to policy enforcement tools.
+Provides authenticatable and auditable information to policy enforcement tools.
 
 ## System Requirements