Fix slotOnload to use the correct on event name (#75)

Author: gabelew

docs/api/ReactGPT.md

@@ -25,7 +25,7 @@ A React component which renders [GPT](https://support.google.com/dfp_sb/answer/1
 - `onSlotRenderEnded`(optional) - An optional event handler function for `googletag.events.SlotRenderEndedEvent`.
 - `onImpressionViewable`(optional) - An optional event handler function for `googletag.events.ImpressionViewableEvent`.
 - `onSlotVisibilityChanged`(optional) - An optional event handler function for `googletag.events.slotVisibilityChangedEvent`.
-- `onSlotLoad`(optional) - An optional event handler function for `googletag.events.SlotOnloadEvent`.
+- `onSlotOnload`(optional) - An optional event handler function for `googletag.events.SlotOnloadEvent`.
 - `renderWhenViewable`(optional) - An optional flag to indicate whether an ad should only render when it's fully in the viewport area.
 - `viewableThreshold`(optional) - An optional number to indicate how much percentage of an ad area needs to be in a viewable area before rendering. Acceptable range is between `0` and `1`.
 - `onScriptLoaded`(optional) - An optional call back function to notify when the script is loaded.

src/Bling.js

@@ -158,9 +158,9 @@ class Bling extends Component {
         /**
          * An optional event handler function for `googletag.events.SlotOnloadEvent`.
          *
-         * @property onSlotLoad
+         * @property onSlotOnload
          */
-        onSlotLoad: PropTypes.func,
+        onSlotOnload: PropTypes.func,
         /**
          * An optional flag to indicate whether an ad should only render when it's fully in the viewport area.
          *