bug#71416: Dead link in the Guile manual

[Yuval Langer]

In the documentation page:

https://www.gnu.org/software/guile/manual/html_node/Syntax-Rules.html#Further-Information

the link to "Joe Marshall’s syntax-rules Primer for the Merely
Eccentric" is dead:

https://sites.google.com/site/evalapply/eccentric.txt

There's an extant (and cleaned up by Wolfgang Corcoran-Mathe, a.k.a.
Zipheir) version of the document on:

https://www.sigwinch.xyz/misc/jrms_syntax-rules_primer.txt

Another version:

https://hipster.home.xs4all.nl/lib/scheme/gauche/define-syntax-primer.txt

Thank you,
Yuval Langer.