Fix typo in Google Maps API externs

This PR fixes a trivial typo in the Google Maps API v3 externs. This avoids the warning:
```
closure-compiler/contrib/externs/maps/google_maps_api_v3.js:1682: WARNING - parameter zIndex does not appear in google.maps.KmlLayer.prototype.setZIndex's parameter list
google.maps.KmlLayer.prototype.setZIndex = function(The) {};
                                           ^
```