Update in Variant spec to decimal table

### Describe the enhancement requested

Minor update in Variant spec (https://github.com/apache/parquet-format/blob/master/VariantEncoding.md):

<img width="520" height="233" alt="Image" src="https://github.com/user-attachments/assets/2fae152e-b8f6-4f70-9336-a834e3da5f12" />

`18 <= precision <= 38` needs to be updated to `19 <= precision <= 38`. 