Update principal-credential-schema.md

aw-design-diagram/principal-credential-schema.md

-**PrincipalMembershipCredential**
+## Principal Credential Schema
 This schema is used to issue VC to Principal User at the time of registration. This VC will indicate respective principal user is part of Gaiax ecosystem. This VC will also be used every time to make respective connection as Trusted via Proof Verification of this schemaID
 
 Following attributes should be in schema (case sensitive)
-
-    1. email // principalUser email address
-    2. issuerDID // issuer OCM organization public DID 
-    3. subjectDID // holder pairwise DID
+```
+ 1. email // principalUser email address
+ 2. issuerDID // issuer OCM organization public DID 
+ 3. subjectDID // holder pairwise DID
\ No newline at end of file
+ ```