docs/alternator/alternator.md: fix link format
docs/alternator/alternator.md: fix link format

Unfortunately the scylla.docs.scylladb.com formatter which generates
https://scylla.docs.scylladb.com/master/alternator/alternator.html
doesn't know how to recognize HTTP URLs and convert them into proper
HTML links (something which github's formatter does).

So convert the two URLs we had in alternator.md into markdown links
which both github and our formatter recognize.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla