Fix minor typos and file name in contributing docs (#1651)

# Description

*Please include a summary of the changes and the related issue. Please
also include relevant motivation and context,
including:*

- What does this PR do?

This PR fixes two minor typos in the contributing top level docs, and
fixes a broken link to another file.

- Why are these changes needed?

While reading the guidelines I noticed two small typos and a broken link
to the documentation guidelines in the same directory.

- How were these changes implemented and what do they affect?

Both typos were fixed. The file has been renamed from
`DOCUMENTATION_GUIDELINE.md` to `DOCUMENTATION_GUIDELINES.md` to match
the link, as the link seems correct vs the filename. There are no other
references to this file within this repo.

There are no open issues related to this