Missing link in standards doc

[rweigel]

At https://github.com/heliophysicsPy/standards/blob/master/standards.md, there are two occurrences of `[link]'.

Also, at https://zenodo.org/record/2529131#.Xvu-x5NKjUI, there is no PDF file, only a .md file.

[jibarnum]

@sapols @jtniehof make sure if we port over standards current wording into new PHEPs that we catch these.