perf kallsyms: Fix the usage on the man page

First, all man pages highlight only perf and subcommands except 'perf
kallsyms', which includes the full usage. Fix it for commands to
monopolize underlines.

Second, options can be ommited when executing 'perf kallsyms', so add
square brackets between <option>.

Signed-off-by: Sangwon Hong <qpakzk@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Taeung Song <treeze.taeung@gmail.com>
Link: http://lkml.kernel.org/r/1518377864-20353-1-git-send-email-qpakzk@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-kallsyms.txt

@@ -8,7 +8,7 @@ perf-kallsyms - Searches running kernel for symbols
 SYNOPSIS
 --------
 [verse]
-'perf kallsyms <options> symbol_name[,symbol_name...]'
+'perf kallsyms' [<options>] symbol_name[,symbol_name...]
 
 DESCRIPTION
 -----------