[SPARK-14520][SQL] Use correct return type in VectorizedParquetInputFormat

## What changes were proposed in this pull request?
JIRA: https://issues.apache.org/jira/browse/SPARK-14520

`VectorizedParquetInputFormat` inherits `ParquetInputFormat` and overrides `createRecordReader`. However, its overridden `createRecordReader` returns a `ParquetRecordReader`. It should return a `RecordReader`. Otherwise, `ClassCastException` will be thrown.

## How was this patch tested?

Existing tests.

Author: Liang-Chi Hsieh <simonh@tw.ibm.com>

Closes #12292 from viirya/fix-vectorized-input-format.