Rename diagram files to align with terminology.

Author: msporny

diagrams/credential.svg renamed to diagrams/vc.svg

@@ -716,7 +716,7 @@ <h3>Credentials</h3>
 
         <figure id="basic-vc">
           <img style="margin: auto; display: block; width: 50%;"
-               src="diagrams/credential.svg" alt="a Verifible
+               src="diagrams/vc.svg" alt="a Verifible
                Credential contains Credential Metadata, Claim(s), and
                Proof(s)">
           <figcaption style="text-align: center;">
@@ -751,7 +751,7 @@ <h3>Credentials</h3>
 
         <figure id="info-graph-vc">
           <img style="margin: auto; display: block; width: 100%;"
-               src="diagrams/credential-graph.svg" alt="diagram with a
+               src="diagrams/vc-graph.svg" alt="diagram with a
                Credential Graph on top connected via a proof to a
                Proof Graph on the bottom.  The Credental Graph has
                Credential 123 with 4 properties: 'type' of value
@@ -845,7 +845,7 @@ <h3>Presentations</h3>
 
         <figure id="info-graph-vp">
           <img style="margin: auto; display: block; width: 75%;"
-               src="diagrams/presentation-graph.svg" alt="diagram with
+               src="diagrams/vp-graph.svg" alt="diagram with
                a Presentation Graph on top connected via a proof to a
                Presentation Proof Graph on the bottom.  The
                Presentation Graph has Presentation ABC with 3