add default ttl to s2sconfig (prebid#2092)

* Update publisher-api-reference.md

documents prebid/Prebid.js#5419

* Update publisher-api-reference.md

Author: patmmccann

dev-docs/publisher-api-reference.md

@@ -1732,6 +1732,7 @@ The `s2sConfig` properties:
 | `endpoint` | Required | URL | Defines the auction endpoint for the Prebid Server cluster |
 | `syncEndpoint` | Required | URL | Defines the cookie_sync endpoint for the Prebid Server cluster |
 | `userSyncLimit` | Optional | Integer | Max number of userSync URLs that can be executed by Prebid Server cookie_sync per request.  If not defined, PBS will execute all userSync URLs included in the request. |
+| `defaultTtl` | Optional | Integer | Configures the default TTL in the Prebid Server adapter to use when Prebid Server does not return a bid TTL - 60 if not set |
 | `adapterOptions` | Optional | Object | Arguments will be added to resulting OpenRTB payload to Prebid Server in every impression object at request.imp[].ext.BIDDER. See the example above. |
 | `extPrebid` | Optional | Object | Arguments will be added to resulting OpenRTB payload to Prebid Server in request.ext.prebid. See the examples below. |
 | `syncUrlModifier` | Optional | Object | Function to modify a bidder's sync url before the actual call to the sync endpoint. Bidder must be enabled for s2sConfig. |