Update mediaTypes description

dev-docs/adunit-reference.md

@@ -37,7 +37,7 @@ See the table below for the list of properties on the ad unit.  For example ad u
 | `code`       | Required | String                                | Unique identifier you create and assign to this ad unit.  Used to set query string targeting on the ad. If using GPT, we recommend setting this to slot element ID.                        |
 | `sizes`      | Required | Array[Number] or Array[Array[Number]] | All sizes this ad unit can accept.  Examples: `[400, 600]`, `[[300, 250], [300, 600]]`.  For 1.0 and later, define sizes within the appropriate `mediaTypes.{banner,native,video}` object. |
 | `bids`       | Required | Array[Object]                         | Array of bid objects representing demand partners and associated parameters for a given ad unit.  See [Bids](#adUnit.bids) below.                                                          |
-| `mediaTypes` | Optional | Object                                | Defines the media type of the ad.  For a list of properties, see [`adUnit.mediaTypes`](#adUnit.mediaTypes) below.                                                                          |
+| `mediaTypes` | Optional | Object                                | Defines one or more media types that can serve into the ad unit.  For a list of properties, see [`adUnit.mediaTypes`](#adUnit.mediaTypes) below.                                                   |
 | `labelAny`   | Optional | Array[String]                         | Used for [conditional ads][conditionalAds].  Works with `sizeConfig` argument to [pbjs.setConfig][setConfig].                                                                              |
 | `labelAll`   | Optional | Array[String]                         | Used for [conditional ads][conditionalAds]. Works with `sizeConfig` argument to [pbjs.setConfig][setConfig].                                                                               |