doc: replay: use a nested definition list

doc: replay: use a nested definition list

This bullet list for `--ref-action` introduces a term with a colon.
This is exactly what a definition list is, structurally. Let’s be
sylistically consistent and use the definition list markup construct.

We can reuse the `::` delimiter since we use an open block.
But for consistency use the typical nested definition list
delimiter, namely `;;`.

Also drop the harmless but unneeded indentation.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>