Fix runtime exception in MapKeyToVector sample. (#3287)

Author: codemzs

docs/samples/Microsoft.ML.Samples/Dynamic/Transforms/Conversion/MapKeyToVector.cs

@@ -67,8 +67,8 @@ private class DataPoint
 
         private class TransformedData : DataPoint
         {
-            public uint[] TimeframeVector { get; set; }
-            public uint[] CategoryVector { get; set; }
+            public float[] TimeframeVector { get; set; }
+            public float[] CategoryVector { get; set; }
         }
     }
 }