ACPI / Kconfig: Update ACPI_PROCFS_POWER help text

Fix grammar and punctuation (end sentences with a period) in the
Kconfig help text for ACPI_PROCFS_POWER.

I was looking at this since it appears to be going away (again,
some day) and I have a working script that uses this info to tell me
battery usage. I can update the script to use /sys/class/power_supply
(in theory) but the contents (with units) should be documented in
Documentation/ABI/ before /proc/acpi/battery/ is removed (IMO).

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>