perf script python: Fix unintended underline

The text ranging from "subsystem__event_name" to "raw_syscalls__sys_enter()"
is interpreted by asciidoc as a pair of unconstrained text formatting markers.

The result is that the manual page displayed this text as underlined,
and the HTML pages displayed this text as italicized. Escape the first
double-underscore to prevent this.

https://docs.asciidoctor.org/asciidoc/latest/syntax-quick-reference/Signed-off-by: NStephen Brennan <stephen.s.brennan@oracle.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210806204502.110305-1-stephen.s.brennan@oracle.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-script-python.txt

@@ -167,7 +167,7 @@ below).
 
 Following those are the 'event handler' functions generated one for
 every event in the 'perf record' output.  The handler functions take
-the form subsystem__event_name, and contain named parameters, one for
+the form subsystem\__event_name, and contain named parameters, one for
 each field in the event; in this case, there's only one event,
 raw_syscalls__sys_enter().  (see the EVENT HANDLERS section below for
 more info on event handlers).