libtraceevent: Fix typo in tep_plugin_add_option() description

A typo "optiona" -> "optional" is fixed in description of
tep_plugin_add_option() API.

Link: https://lore.kernel.org/r/CAM9d7cizjF+fbK7YzmsBDgrx__4YAOsmEq67D3sWET8FF+YdFA@mail.gmail.com
Link: https://lore.kernel.org/linux-trace-devel/20200714103027.2477584-4-tz.stoyanov@gmail.com
Link: https://lore.kernel.org/linux-trace-devel/20200716092014.2613403-4-tz.stoyanov@gmail.comSuggested-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/lkml/20200722011755.014613924@goodmis.orgSigned-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/traceevent/event-plugin.c

@@ -310,7 +310,7 @@ static int process_option(const char *plugin, const char *option, const char *va
 /**
  * tep_plugin_add_option - add an option/val pair to set plugin options
  * @name: The name of the option (format: <plugin>:<option> or just <option>)
- * @val: (optiona) the value for the option
+ * @val: (optional) the value for the option
  *
  * Modify a plugin option. If @val is given than the value of the option
  * is set (note, some options just take a boolean, so @val must be either