patchdiff-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>
> ---
>  The error mode changes between 2.49.0 and 2.50.0-rc0, causing this to
>  stand out in "doc-diff". But this is strictly speaking not a regression
>  fix for 2.50.
> 
>  Documentation/diff-generate-patch.adoc | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/Documentation/diff-generate-patch.adoc b/Documentation/diff-generate-patch.adoc
> index e5c813c96f..7b6cdd1980 100644
> --- a/Documentation/diff-generate-patch.adoc
> +++ b/Documentation/diff-generate-patch.adoc
> @@ -138,7 +138,7 @@ or like this (when the `--cc` option is used):
>  +
>  [synopsis]
>  index <hash>,<hash>..<hash>
> -mode <mode>,<mode>`..`<mode>
> +mode <mode>,<mode>..<mode>
>  new file mode <mode>
>  deleted file mode <mode>,<mode>
>  +