PARQUET-1862: fix comment mistake of DataPageHeaderV2

[Liam8]

Statistics in DataPageHeaderV2 should be about the page, not column chunk.

Make sure you have checked all steps below.

Jira

• My PR addresses the following Parquet Jira issues and references them in the PR title. For example, "PARQUET-1234: My Parquet PR"
  • https://issues.apache.org/jira/browse/PARQUET-XXX
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Commits

• My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain Javadoc that explain what it does