diff-generate-patch.adoc: drop spurious backticks

diff-generate-patch.adoc: drop spurious backticks

Commit 0b080a70ab (doc: git-diff: apply format changes to
diff-generate-patch, 2024-11-18) wrapped the ".." in

  mode <mode>,<mode>..<mode>

in backticks. Note how the line before is quite similar,

  index <hash>,<hash>..<hash>

but did not get any backticks. Remove the backticks, since they confuse
Asciidoctor.

The exact failure mode changed with c87b2b3a6f (doc: fix asciidoctor
synopsis processing of triple-dots, 2025-04-12), and arguably to the
better. But Asciidoctor (2.0.18) still ends up confused by these
backticks and leaves the manpage rendering as

  index <hash>,<hash>..<hash>
  mode <mode>,<mode>`..__<mode>__
  {empty}`new file mode <mode>

Drop the backticks. This is a no-op with asciidoc (10.2.0).

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>