Add commas suggested by @agl

Co-Authored-By: Adam Langley <agl@google.com>

index.bs

@@ -5858,7 +5858,7 @@ the wrong [=credential ID=], or if an attacker intercepts and manipulates the [=
 ## Physical Proximity between Client and Authenticator ## {#sctn-client-authenticator-proximity}
 
 In the WebAuthn [=authenticator model=], it is generally assumed that [=roaming authenticators=]
-are physically close to and communicate directly with the [=client=].
+are physically close to, and communicate directly with, the [=client=].
 This arrangement has some important advantages.
 
 The promise of physical proximity between [=client=] and [=authenticator=]