perf list: Unify metric group description format with PMU event description

perf list: Unify metric group description format with PMU event description

PMU event descriptions use 7 spaces + '[' or 8 spaces as indentation.
Metric groups used a tab + '['. This patch unifies it to the way PMU
event descriptions are indented.

BEFORE:

  $ perf list
  [...]
  Metric Groups:

  DSB:
    DSB_Coverage
	  [Fraction of Uops delivered by the DSB (aka Decoded Icache; or Uop Cache)]
  [...]

AFTER:

  $ perf list
  [...]
  Metric Groups:

  DSB:
    DSB_Coverage
         [Fraction of Uops delivered by the DSB (aka Decoded Icache; or Uop Cache)]
  [...]

Signed-off-by: Michael Petlan <mpetlan@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Kim Phillips <kim.phillips@arm.com>
LPU-Reference: 771439042.22924766.1532986504631.JavaMail.zimbra@redhat.com
Link: https://lkml.kernel.org/n/tip-mlo850517m6u1rbjndvd1bwr@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>