Fix typo in federated_core.md: 'type od' -> 'type of'
@@ -299,7 +299,7 @@ public API:
For example:
* `add_up_integeres(x)` represents an invocation of the TensorFlow
- computation defined earlier on an argument `x`. The type od this
+ computation defined earlier on an argument `x`. The type of this
expression is `int32`.
* `tff.federated_mean(sensor_readings)` represents an invocation of the
