Documentation: trace: histogram: Convert ftrace docs cross-reference

Documentation: trace: histogram: Convert ftrace docs cross-reference

In brief "Extended error information" section, details on error
condition is referred to ftrace docs, which is written in :file:
directive instead of a proper cross-reference. Convert it.

Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20250916054202.582074-7-bagasdotme@gmail.com>