The English Wikipedia edited the message
https://en.wikipedia.org/wiki/MediaWiki:Cite_references_link_many_format.
By default, this message is:
<sup>[[#$1|$2]]</sup>
You can edit your wiki's
[[MediaWiki:Cite_references_link_many_format]] (create it if it
doesn't exist) and change the $2 to $3. This replaces the numbers
(1.1, 1.2, and so on) with letters (a, b, and so on).
Best
Thiemo