tracing/eprobe: Fix to add filter on eprobe description in README file

Fix to add a description of the filter on eprobe in README file. This
is required to identify the kernel supports the filter on eprobe or not.

Link: https://lore.kernel.org/all/167309833728.640500.12232259238201433587.stgit@devnote3/

Fixes: 752be5c ("tracing/eprobe: Add eprobe filter support")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: mhiramat

kernel/trace/trace.c

@@ -5598,7 +5598,7 @@ static const char readme_msg[] =
 #ifdef CONFIG_HIST_TRIGGERS
 	"\t           s:[synthetic/]<event> <field> [<field>]\n"
 #endif
-	"\t           e[:[<group>/][<event>]] <attached-group>.<attached-event> [<args>]\n"
+	"\t           e[:[<group>/][<event>]] <attached-group>.<attached-event> [<args>] [if <filter>]\n"
 	"\t           -:[<group>/][<event>]\n"
 #ifdef CONFIG_KPROBE_EVENTS
 	"\t    place: [<module>:]<symbol>[+<offset>]|<memaddr>\n"