Add missing import to the sample in README.md

[lupino3]

With the latest Parquet.Net version, the sample does not compile out of the box, because a couple of imports is missing.

Add the missing Parquet import and use type inference to work around the missing import for Array.

Description

desctiption goes here

• [ ] I have included unit tests validating this fix.
• [ ] I have updated markdown documentation where required.
• [x] I understand that successful approval of my pull request requires reproducible tests as per Contribution Guideline.