Update derived-metrics.md (#4044)

removing 'This can't be used with `offset_to_grain`.' as it's not
applicable now

website/docs/docs/build/derived-metrics.md

@@ -21,7 +21,7 @@ In MetricFlow, derived metrics are metrics created by defining an expression usi
 | `metrics` |  The list of metrics used in the derived metrics. | Required  |
 | `alias` | Optional alias for the metric that you can use in the expr. | Optional |
 | `filter` | Optional filter to apply to the metric. | Optional |
-| `offset_window` | Set the period for the offset window, such as 1 month. This will return the value of the metric one month from the metric time. This can't be used with `offset_to_grain`. | Required |
+| `offset_window` | Set the period for the offset window, such as 1 month. This will return the value of the metric one month from the metric time.  | Required |
 
 The following displays the complete specification for derived metrics, along with an example.