perf config: Fix example command in manpage to conform to syntax specified in the SYNOPSIS section.

Committer testing:

With the previously documented example:

  $ perf config --user report sort-order=srcline
  The config variable does not contain a section name: report
  $

With the fixed example line:

  $ perf config --user report.sort-order=srcline
  $ perf config --user report.sort-order
  report.sort-order=srcline
  $
Signed-off-by: Nick Thompson <nathompson7@protonmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/linux-perf-users/20201217142619.GA14524@redhat.com/Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-config.txt

@@ -138,7 +138,7 @@ If you want to add or modify several config items, you can do like
 
 To modify the sort order of report functionality in user config file(i.e. `~/.perfconfig`), do
 
-	% perf config --user report sort-order=srcline
+	% perf config --user report.sort-order=srcline
 
 To change colors of selected line to other foreground and background colors
 in system config file (i.e. `$(sysconf)/perfconfig`), do