Fix formatting of sample exr file in OpenEXRFileLayout.rst

The formatting here is confusing and hard to read: https://openexr.com/en/latest/OpenEXRFileLayout.html#sample-file

The table show simply annotate each byte of the file, alongside the hex value of the byte. The formatting would probably work better as a single long column.

This task requires some basic familiarity with [reStructuredText](https://docutils.sourceforge.io/rst.html) and [sphinx](https://www.sphinx-doc.org/en/master) but the concepts are easy to pick up from other examples. See [Building the Website](https://openexr.com/en/latest/install.html#building-the-website) for how to test the formatting.

Open OpenEXRFileLayout.rst and locate the sample file section linked as “sample-file”; compare its reStructuredText table with other readable examples in the documentation. Follow the Building the Website instructions to build the site and confirm the sample EXR bytes and hex values render clearly in a single long column.

Written by the indexing model from the issue text.