Crosslink documentation fix needed

[andrewrobertjones]

Spotted by Eugen:

I noticed an inconsistency in the specification document on GitHub.
When it was decided to rename the second peptide for cross-links from
"receiver" to "acceptor", not all occurrences of "receiver"
have been updated. In the pdf version those are searchable with text search,
and there are also some in the docx in an embedded image of example mzid
file content and a table on page 34.

[colin-combe]

think this can be closed