Formatting is broken for description of BIT_PACKED

The Nested Encoding section of documentation doesn't escape the `_` character, so it looks as following:

Two encodings for the levels are supported BIT_PACKED and RLE. Only RLE is now used as it supersedes BIT_PACKED.

instead of

Two encodings for the levels are supported BIT_PACKED and RLE. Only RLE is now used as it supersedes BIT_PACKED.

**Reporter**: [Alex Ott](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alexott)

_{**Note**: *This issue was originally created as [PARQUET-2085](https://issues.apache.org/jira/browse/PARQUET-2085). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Open the documentation's Nested Encoding section and inspect the sentence describing BIT_PACKED and RLE. Escape the underscore in BIT_PACKED so it renders as intended, then verify the displayed wording matches the issue's expected text.

Written by the indexing model from the issue text.