Fixed typo in exception message

Changed "Only one operation can be active at at time" to "Only one operation can be active at a time"

src/Media.Plugin/iOS/MediaImplementation.cs

@@ -271,7 +271,7 @@ private Task<MediaFile> GetMediaAsync(UIImagePickerControllerSourceType sourceTy
             MediaPickerDelegate ndelegate = new MediaPickerDelegate(viewController, sourceType, options);
             var od = Interlocked.CompareExchange(ref pickerDelegate, ndelegate, null);
             if (od != null)
-                throw new InvalidOperationException("Only one operation can be active at at time");
+                throw new InvalidOperationException("Only one operation can be active at a time");
 
             var picker = SetupController(ndelegate, sourceType, mediaType, options);