Brightness: Exponent: fix default value typo →4

The default value specified on the manual is 4. This gives a rather
smooth brightness change following our own visual perception
(luminance/brightness curve). Litterature shows a value between 3-4.

However, a typo in the code set the default value 1 instead of 4, making the steps appear
non linear.

This fixes that

Signed-off-by: Frank Villaro-Dixon <[email protected]>

brightnessctl.c

@@ -132,7 +132,7 @@ int main(int argc, char **argv) {
  fail("Unable to determine current OS. Exiting!\n");
  if (strcmp(name.sysname, "Linux"))
  fail("This program only supports Linux.\n");
- p.exponent = 1;
+ p.exponent = 4;
  p.min = (struct value){ .val = 0, .v_type = ABSOLUTE, .d_type = DIRECT, .sign = PLUS };
  while ((c = getopt_long(argc, argv, "lqpmPn::e::srhVc:d:", options, NULL)) >= 0) {
  switch (c) {