Fix grammar on watermarking proof definition.

Co-authored-by: Dave Longley <[email protected]>

index.html

@@ -2026,7 +2026,7 @@ <h3>Securing Verifiable Credentials</h3>
 <dfn class="export">enveloping proof</dfn> is one that wraps an expression of
 this data model, such as via a JSON Web Token, which is elaborated on in the
 <em>Securing Verifiable Credentials using JOSE and COSE</em> [[VC-JOSE-COSE]] specification.
-An <dfn class="export">watermarking proof</dfn> is a mechanism where the proof is
+A <dfn class="export">watermarking proof</dfn> is a mechanism where the proof is
 included in the data model, such as a Data Integrity Proof, which is elaborated
 on in <em>Verifiable Credential Data Integrity</em> [[VC-DATA-INTEGRITY]].
         </p>