doc: format-rev: use [synopsis] on code block

This code block uses the placeholder `<subject>`. Let’s highlight this
placeholder properly by using the `synopsis` open block definition which
was introduced in a34d1d53 (doc: convert git-show to synopsis style,
2026-02-06). This renders the block like a code block but with emphasis
styling on placeholders, just like inline-verbatim (`) in running text.

Yes, note that open blocks since commit a34d1d53 can, on synopsis-style
docs like this one, be immediately preceded by `[synopsis]`, just like
the command synopsis is:

    [synopsis]
    (EXPERIMENTAL!) git format-rev - [...]

Cf. verse-style:

    [verse]
    'git name-rev' [...]

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

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