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 <[e-mail]>
Signed-off-by: Bagas Sanjaya <[e-mail]>
Acked-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Message-ID: <[e-mail]>

Documentation/trace/histogram.rst

···
 
   For some error conditions encountered when invoking a hist trigger
   command, extended error information is available via the
-  tracing/error_log file.  See Error Conditions in
-  :file:`Documentation/trace/ftrace.rst` for details.
+  tracing/error_log file.  See "Error conditions" section in
+  Documentation/trace/ftrace.rst for details.
 
 2.3. 'hist' trigger examples
 ----------------------------