Fix link to parquet-mr project in README.md Contributing section

[asfimport]

There's a broken link inFix link to parquet-mr project in README.md Contributing section which needs fixing (github.com/Parquet/parquet-mr).

Reporter: Lars Volker / @lekv
Assignee: Anna Szonyi

Related issues:

• Wrong and broken links in README (is duplicated by)

PRs and other links:

• Pull request

_{Note: This issue was originally created as PARQUET-1034. Please see the migration documentation for further details.}