Minor punctuation fix to controller document language.

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

index.html

@@ -1286,7 +1286,7 @@ <h2>Relationship to Verifiable Credentials</h2>
 [=verification method=] are expressed using the `revocation` and `expires`
 properties, respectively; are related to events such as a private key being
 compromised or expiring; and can provide timing information which might reveal
-details about a controller such as their security practices or when they might
+details about a controller, such as their security practices or when they might
 have been compromised. The revocation information for a [=verifiable
 credential=] is expressed using the `credentialStatus` property; is related
 to events such as an individual losing the privilege that is granted by the