perf script python: Fix unintended underline

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: Stephen 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.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>