docs/trace: fix a label of boottime-trace

Commit 55978953 ("Documentation: tracing: Add histogram syntax to
boot-time tracing") introduced a warning:

linux/Documentation/trace/boottime-trace.rst:136: WARNING: undefined label: histogram (if the link has no caption the label must precede a section header)

Replace with: (path)
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 55978953 ("Documentation: tracing: Add histogram syntax to boot-time tracing")
Link: https://lore.kernel.org/r/20211210041536.1446734-1-siyanteng@loongson.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/boottime-trace.rst

@@ -131,9 +131,7 @@ Ftrace Histogram Options
 Since it is too long to write a histogram action as a string for per-event
 action option, there are tree-style options under per-event 'hist' subkey
 for the histogram actions. For the detail of the each parameter,
-please read the event histogram document [3]_.
-
-.. [3] See :ref:`Documentation/trace/histogram.rst <histogram>`
+please read the event histogram document (Documentation/trace/histogram.rst)
 
 ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist.[N.]keys = KEY1[, KEY2[...]]
   Set histogram key parameters. (Mandatory)