Fix parquet-cli's wrong or missing usage examples

1. The following parquet-cli's `to-avro` usage examples fail due to the lack of `-o` options.
In addition, "sample.parquet" in the second example should be "sample.avro".

```Java

Examples:

# Create an Avro file from a Parquet file
parquet to-avro sample.parquet sample.avro

# Create an Avro file in HDFS from a local JSON file
parquet to-avro path/to/sample.json hdfs:/user/me/sample.parquet

# Create an Avro file from data in S3
parquet to-avro s3:/data/path/sample.parquet sample.avro
```

2. The above is the same for convert-csv.

```Java

Examples:

# Create a Parquet file from a CSV file
parquet convert-csv sample.csv sample.parquet --schema schema.avsc

# Create a Parquet file in HDFS from local CSV
parquet convert-csv path/to/sample.csv hdfs:/user/me/sample.parquet --schema schema.avsc

# Create an Avro file from CSV data in S3
parquet convert-csv s3:/data/path/sample.csv sample.avro --format avro --schema s3:/schemas/schema.avsc
```

3. The meta command has an "Examples:" heading but lacks its content.

```Java

$ java -cp 'target/*:target/dependency/*' org.apache.parquet.cli.Main help meta

Usage: parquet [general options] meta [command options]

Description:

Print a Parquet file's metadata

Examples:

```

**Reporter**: [Kengo Seki](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=sekikn) / @sekikn
**Assignee**: [Kengo Seki](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=sekikn) / @sekikn
#### PRs and other links:
- [GitHub Pull Request #649](https://github.com/apache/parquet-mr/pull/649)

_{**Note**: *This issue was originally created as [PARQUET-1597](https://issues.apache.org/jira/browse/PARQUET-1597). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}