Proof Set and Chain Test Vectors: Wording fix.

Co-authored-by: Ted Thibodeau Jr <[email protected]>

index.html

@@ -2033,7 +2033,7 @@ <h3>Proof Sets and Chains</h3>
           <h4>Proof Set</h4>
           <p>
 To demonstrate creating a proof set, we start with a document containing a
-single proof and add another proof to it. The start document is shown below and
+single proof and add another proof to it. The starting document is shown below and
 contains a proof signed with `keyPair1`.
           </p>
           <pre class="example nohighlight" title="Starting Document for Proof Set" data-include="TestVectors/proof-set-chain/signedProofSet1.json"