samples: Bluetooth: use correct periodic advertising intervals

BT_GAP_ADV_SLOW_INT_MIN and BT_GAP_ADV_SLOW_INT_MAX represent 1s and 1.2s,
respectively, in the extended advertising context, where the bit interval
is 0.625 ms. Using them for periodic advertising will result in a range
of [2s, 2.4s], as the bit interval for periodic advertising is 1.25ms.

Instead, BT_GAP_PER_ADV_SLOW_INT_MIN and BT_GAP_PER_ADV_SLOW_INT_MAX
should be used in this sample, and the range will become [1s, 1.2s].

Signed-off-by: Yuxuan Cai <yuxuan.cai@nordicsemi.no>