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>